fix: Only check keyHasError if ordering key is non-empty (#367)

* feat: Add flow control support to publisher

* make suggested fixes

* chore: Remove note that ordering keys requires enablements.

* feat: Add support for server-side flow control

* Revert "chore: Remove note that ordering keys requires enablements."

This reverts commit 9c113c3.

* fix: Fix import order

* fix: Make error message more clear about where ordering must be enabled when publishing.

* fix: Ensure that messages that are in pending batches for an ordering key are canceled when a previous publish for the ordering keys fails.

* fix: Only check keyHasError if ordering keys is non-empty

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java

@@ -256,7 +256,7 @@ public ApiFuture<String> publish(PubsubMessage message) {
     List<OutstandingBatch> batchesToSend;
     messagesBatchLock.lock();
     try {
-      if (sequentialExecutor.keyHasError(orderingKey)) {
+      if (!orderingKey.isEmpty() && sequentialExecutor.keyHasError(orderingKey)) {
         outstandingPublish.publishResult.setException(
             SequentialExecutorService.CallbackExecutor.CANCELLATION_EXCEPTION);
         return outstandingPublish.publishResult;