[LB policies] remove unnecessary check for null config (#32932)

These checks have not been needed since way back in #22100, but they
were never removed, and they've even propagated to a bunch of new
policies via copy-paste.

src/core/ext/filters/client_channel/lb_policy/outlier_detection/outlier_detection.cc

@@ -992,14 +992,6 @@ class OutlierDetectionLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // This policy was configured in the deprecated loadBalancingPolicy
-      // field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:outlier_detection policy requires "
-          "configuration. Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     ValidationErrors errors;
     OutlierDetectionConfig outlier_detection_config;
     RefCountedPtr<LoadBalancingPolicy::Config> child_policy;

src/core/ext/filters/client_channel/lb_policy/priority/priority.cc

@@ -903,14 +903,6 @@ class PriorityLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // priority was mentioned as a policy in the deprecated
-      // loadBalancingPolicy field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:priority policy requires "
-          "configuration. Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     return LoadRefCountedFromJson<PriorityLbConfig>(
         json, JsonArgs(), "errors validating priority LB policy config");
   }

src/core/ext/filters/client_channel/lb_policy/weighted_round_robin/weighted_round_robin.cc

@@ -993,12 +993,6 @@ class WeightedRoundRobinFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:weighted_round_robin policy "
-          "requires configuration. Please use loadBalancingConfig field of "
-          "service config instead.");
-    }
     return LoadRefCountedFromJson<WeightedRoundRobinConfig>(
         json, JsonArgs(),
         "errors validating weighted_round_robin LB policy config");

src/core/ext/filters/client_channel/lb_policy/xds/cds.cc

@@ -743,13 +743,6 @@ class CdsLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // xds was mentioned as a policy in the deprecated loadBalancingPolicy
-      // field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:cds policy requires configuration. "
-          "Please use loadBalancingConfig field of service config instead.");
-    }
     return LoadRefCountedFromJson<CdsLbConfig>(
         json, JsonArgs(), "errors validating cds LB policy config");
   }

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_impl.cc

@@ -777,14 +777,6 @@ class XdsClusterImplLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // This policy was configured in the deprecated loadBalancingPolicy
-      // field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:xds_cluster_impl policy requires "
-          "configuration. Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     return LoadRefCountedFromJson<XdsClusterImplLbConfig>(
         json, JsonArgs(),
         "errors validating xds_cluster_impl LB policy config");

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_manager.cc

@@ -672,14 +672,6 @@ class XdsClusterManagerLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // xds_cluster_manager was mentioned as a policy in the deprecated
-      // loadBalancingPolicy field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:xds_cluster_manager policy requires "
-          "configuration.  Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     return LoadRefCountedFromJson<XdsClusterManagerLbConfig>(
         json, JsonArgs(),
         "errors validating xds_cluster_manager LB policy config");

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_resolver.cc

@@ -1145,14 +1145,6 @@ class XdsClusterResolverLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // xds_cluster_resolver was mentioned as a policy in the deprecated
-      // loadBalancingPolicy field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:xds_cluster_resolver policy "
-          "requires configuration. "
-          "Please use loadBalancingConfig field of service config instead.");
-    }
     return LoadRefCountedFromJson<XdsClusterResolverLbConfig>(
         json, JsonArgs(),
         "errors validating xds_cluster_resolver LB policy config");

src/core/ext/filters/client_channel/lb_policy/xds/xds_override_host.cc

@@ -740,14 +740,6 @@ class XdsOverrideHostLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // This policy was configured in the deprecated loadBalancingPolicy
-      // field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:xds_override_host policy requires "
-          "configuration. Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     return LoadRefCountedFromJson<XdsOverrideHostLbConfig>(
         json, JsonArgs(),
         "errors validating xds_override_host LB policy config");

src/core/ext/filters/client_channel/lb_policy/xds/xds_wrr_locality.cc

@@ -346,14 +346,6 @@ class XdsWrrLocalityLbFactory : public LoadBalancingPolicyFactory {
 
   absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
   ParseLoadBalancingConfig(const Json& json) const override {
-    if (json.type() == Json::Type::kNull) {
-      // xds_wrr_locality was mentioned as a policy in the deprecated
-      // loadBalancingPolicy field or in the client API.
-      return absl::InvalidArgumentError(
-          "field:loadBalancingPolicy error:xds_wrr_locality policy requires "
-          "configuration.  Please use loadBalancingConfig field of service "
-          "config instead.");
-    }
     return LoadRefCountedFromJson<XdsWrrLocalityLbConfig>(
         json, JsonArgs(),
         "errors validating xds_wrr_locality LB policy config");