NullPointerException when writing an empty table to BigQuery

Originally reported on Stackoverflow:
https://stackoverflow.com/questions/44314030/handling-empty-pcollections-with-bigquery-in-apache-beam

It looks like if there is no data to write, then WritePartitions will return a null destination, as explicitly stated in the comments:

https://github.com/apache/beam/blob/v2.0.0/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/WritePartition.java#L126

But, the ConstantTableDestination doesn't turn that into the constant destination as the comment promises, instead it returns that `null` destination:

https://github.com/apache/beam/blob/53c9bf4cd325035fabde192c63652ef6d591b93c/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/DynamicDestinationsHelpers.java#L74

This leads to a null pointer error here since the `tableDestination` is that null result from calling `getTable`:

https://github.com/apache/beam/blob/v2.0.0/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/WriteTables.java#L97