@Override
          public String apply(KV<String, String> message, KV<String, String> record) {
            joinInvokedLatch.countDown();
            return null;
          }
Ensure join function is not invoked more than once when join function returns null on the first invocation
samza-core/src/test/java/org/apache/samza/operators/impl/TestStreamTableJoinOperatorImpl.java