RATIS-1987. Intermittent NPE in TestGrpcZeroCopy.

[szetszwo]

See RATIS-1987.

The test client should enqueue before calling onNext(..).
After this change, passed 100 times: https://github.com/szetszwo/ratis/actions/runs/7532901374

[adoroszlai]

Thanks @szetszwo for the patch, LGTM.

Should the same fix be applied to helloReplies?

ratis/ratis-test/src/test/java/org/apache/ratis/grpc/util/GrpcZeroCopyTestClient.java

Lines 136 to 143 in e25480b

	final CompletableFuture<String> f = new CompletableFuture<>();
	try {
	helloRequestHandler.onNext(request);
	helloReplies.offer(f);
	} catch (IllegalStateException e) {
	// already closed
	f.completeExceptionally(e);
	}

[szetszwo]

@adoroszlai , Thanks for reviewing this! Yes, we should also fix helloReplies.