GcsUtil can try to execute empty batches in some cases.

I think, but have not confirmed, that this happens because we try to remove an empty set of files.

```

SEVERE: 2016-12-22T16:59:17.063Z: (e72c8b756ce69f1a): java.io.IOException: Error executing batch GCS
request
at org.apache.beam.sdk.util.GcsUtil.executeBatches(GcsUtil.java:486)
at org.apache.beam.sdk.util.GcsUtil.remove(GcsUtil.java:535)

at org.apache.beam.sdk.util.GcsIOChannelFactory.remove(GcsIOChannelFactory.java:108)

at org.apache.beam.sdk.io.FileBasedSink$FileBasedWriteOperation.removeTemporaryFiles(FileBasedSink.java:468)

at org.apache.beam.sdk.io.FileBasedSink$FileBasedWriteOperation.finalize(FileBasedSink.java:382)

at org.apache.beam.sdk.io.Write$Bound$2.processElement(Write.java:417)
Caused by: java.util.concurrent.ExecutionException:
java.lang.IllegalArgumentException: no JSON input found
at org.apache.beam.sdk.repackaged.com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:476)

at org.apache.beam.sdk.repackaged.com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:455)

at org.apache.beam.sdk.repackaged.com.google.common.util.concurrent.AbstractFuture$TrustedFuture.get(AbstractFuture.java:79)

at org.apache.beam.sdk.util.GcsUtil.executeBatches(GcsUtil.java:481)
at org.apache.beam.sdk.util.GcsUtil.remove(GcsUtil.java:535)

at org.apache.beam.sdk.util.GcsIOChannelFactory.remove(GcsIOChannelFactory.java:108)

at org.apache.beam.sdk.io.FileBasedSink$FileBasedWriteOperation.removeTemporaryFiles(FileBasedSink.java:468)

at org.apache.beam.sdk.io.FileBasedSink$FileBasedWriteOperation.finalize(FileBasedSink.java:382)

at org.apache.beam.sdk.io.Write$Bound$2.processElement(Write.java:417)
at org.apache.beam.sdk.io.Write$Bound$2$auxiliary$3HTfRVcv.invokeProcessElement(Unknown
Source)
at org.apache.beam.sdk.transforms.DoFnAdapters$SimpleDoFnAdapter.processElement(DoFnAdapters.java:234)

at org.apache.beam.runners.core.SimpleOldDoFnRunner.invokeProcessElement(SimpleOldDoFnRunner.java:118)

at org.apache.beam.runners.core.SimpleOldDoFnRunner.processElement(SimpleOldDoFnRunner.java:104)

at com.google.cloud.dataflow.worker.runners.worker.SimpleParDoFn.processElement(SimpleParDoFn.java:192)

```

Imported from Jira [BEAM-1209](https://issues.apache.org/jira/browse/BEAM-1209). Original Jira may contain additional context.
Reported by: dhalperi.

Start in GcsUtil.java at executeBatches and remove, using the stack trace to follow the cleanup path through GcsIOChannelFactory.java and FileBasedSink.java. Confirm how an empty removal set reaches batch execution; done means empty batches no longer trigger the reported GCS error, with regression coverage for that path.

Written by the indexing model from the issue text.