core: fix NPE in ConfigSelectingClientCall

Fix the following bug:

ManagedChannelImpl.ConfigSelectingClientCall may return early in start() leaving delegate null, and fails request() method after start().

Currently the bug can only be triggered when using xDS.

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -1179,13 +1179,15 @@ protected ClientCall<ReqT, RespT> delegate() {
       return delegate;
     }
 
+    @SuppressWarnings("unchecked")
     @Override
     public void start(Listener<RespT> observer, Metadata headers) {
       PickSubchannelArgs args = new PickSubchannelArgsImpl(method, headers, callOptions);
       InternalConfigSelector.Result result = configSelector.selectConfig(args);
       Status status = result.getStatus();
       if (!status.isOk()) {
         executeCloseObserverInContext(observer, status);
+        delegate = (ClientCall<ReqT, RespT>) NOOP_CALL;
         return;
       }
       ClientInterceptor interceptor = result.getInterceptor();
@@ -1226,6 +1228,29 @@ public void cancel(@Nullable String message, @Nullable Throwable cause) {
     }
   }
 
+  private static final ClientCall<Object, Object> NOOP_CALL = new ClientCall<Object, Object>() {
+    @Override
+    public void start(Listener<Object> responseListener, Metadata headers) {}
+
+    @Override
+    public void request(int numMessages) {}
+
+    @Override
+    public void cancel(String message, Throwable cause) {}
+
+    @Override
+    public void halfClose() {}
+
+    @Override
+    public void sendMessage(Object message) {}
+
+    // Always returns {@code false}, since this is only used when the startup of the call fails.
+    @Override
+    public boolean isReady() {
+      return false;
+    }
+  };
+
   /**
    * Terminate the channel if termination conditions are met.
    */

core/src/test/java/io/grpc/internal/ConfigSelectingClientCallTest.java

@@ -135,6 +135,9 @@ public Result selectConfig(PickSubchannelArgs args) {
     ArgumentCaptor<Status> statusCaptor = ArgumentCaptor.forClass(null);
     verify(callListener).onClose(statusCaptor.capture(), any(Metadata.class));
     assertThat(statusCaptor.getValue().getCode()).isEqualTo(Status.Code.FAILED_PRECONDITION);
+
+    // The call should not delegate to null and fail methods with NPE.
+    configSelectingClientCall.request(1);
   }
 
   private final class TestChannel extends Channel {