Fixes a bug where KStream#transformValues would forward null values from the provided ValueTransform#transform operation.
A test was added for verification KStreamTransformValuesTest#shouldEmitNoRecordIfTransformReturnsNull. A parallel test for non-key ValueTransformer was not added, as they share the same code path.
Reviewers: Bruno Cadonna <bruno@confluent.io>, Bill Bejeck <bbejeck@gmail.com>