[lb_policy] Remove 2 nullptr checks for `endpoint_list_` in round_robin and wrr (#37685)

Closes #37673

Closes #37685

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37685 from yijiem:nullptr-check-lb fecac2b751
PiperOrigin-RevId: 677872993

src/core/load_balancing/round_robin/round_robin.cc

@@ -361,14 +361,10 @@ void RoundRobin::RoundRobinEndpointList::
        (num_ready_ > 0 && AllEndpointsSeenInitialState()) ||
        num_transient_failure_ == size())) {
     if (GRPC_TRACE_FLAG_ENABLED(round_robin)) {
-      const std::string old_counters_string =
-          round_robin->endpoint_list_ != nullptr
-              ? round_robin->endpoint_list_->CountersString()
-              : "";
       LOG(INFO) << "[RR " << round_robin << "] swapping out child list "
                 << round_robin->endpoint_list_.get() << " ("
-                << old_counters_string << ") in favor of " << this << " ("
-                << CountersString() << ")";
+                << round_robin->endpoint_list_->CountersString()
+                << ") in favor of " << this << " (" << CountersString() << ")";
     }
     round_robin->endpoint_list_ =
         std::move(round_robin->latest_pending_endpoint_list_);

src/core/load_balancing/weighted_round_robin/weighted_round_robin.cc

@@ -947,12 +947,10 @@ void WeightedRoundRobin::WrrEndpointList::
        (num_ready_ > 0 && AllEndpointsSeenInitialState()) ||
        num_transient_failure_ == size())) {
     if (GRPC_TRACE_FLAG_ENABLED(weighted_round_robin_lb)) {
-      const std::string old_counters_string =
-          wrr->endpoint_list_ != nullptr ? wrr->endpoint_list_->CountersString()
-                                         : "";
       LOG(INFO) << "[WRR " << wrr << "] swapping out endpoint list "
-                << wrr->endpoint_list_.get() << " (" << old_counters_string
-                << ") in favor of " << this << " (" << CountersString() << ")";
+                << wrr->endpoint_list_.get() << " ("
+                << wrr->endpoint_list_->CountersString() << ") in favor of "
+                << this << " (" << CountersString() << ")";
     }
     wrr->endpoint_list_ = std::move(wrr->latest_pending_endpoint_list_);
   }