xds_client_fuzzer: fix send message on a stream that does not exist yet (#31648)

test/core/xds/xds_client_corpora/send_message_to_client_before_stream_created

@@ -0,0 +1,9 @@
+bootstrap: "{\"xds_servers\": [{\"server_uri\":\"xds.example.com:443\", \"channel_creds\":[{\"type\": \"fake\"}]}]}"
+actions {
+  send_message_to_client {
+    stream_id {
+      ads {
+      }
+    }
+  }
+}

test/core/xds/xds_transport_fake.cc

@@ -224,6 +224,7 @@ FakeXdsTransportFactory::Create(
 void FakeXdsTransportFactory::TriggerConnectionFailure(
     const XdsBootstrap::XdsServer& server, absl::Status status) {
   auto transport = GetTransport(server);
+  if (transport == nullptr) return;
   transport->TriggerConnectionFailure(std::move(status));
 }
 
@@ -237,15 +238,14 @@ FakeXdsTransportFactory::WaitForStream(const XdsBootstrap::XdsServer& server,
                                        const char* method,
                                        absl::Duration timeout) {
   auto transport = GetTransport(server);
+  if (transport == nullptr) return nullptr;
   return transport->WaitForStream(method, timeout);
 }
 
 RefCountedPtr<FakeXdsTransportFactory::FakeXdsTransport>
 FakeXdsTransportFactory::GetTransport(const XdsBootstrap::XdsServer& server) {
   MutexLock lock(&mu_);
-  RefCountedPtr<FakeXdsTransport> transport = transport_map_[&server];
-  GPR_ASSERT(transport != nullptr);
-  return transport;
+  return transport_map_[&server];
 }
 
 }  // namespace grpc_core