Exception raised when using MongoDBIO.Write in streaming mode

In org.apache.beam.sdk.io.mongodb.MongoDbIO.Write.WriteFn#flush(), no check is done on the size of the batch list of documents before executing the insertMany operation.

In streaming mode, when processing an empty pane, an empty list of documents can be passed to the MongoDB client which results in the following exception:

java.lang.IllegalArgumentException: state should be: writes is not an empty list
at com.mongodb.assertions.Assertions.isTrueArgument(Assertions.java:99)
at com.mongodb.operation.MixedBulkWriteOperation.<init>(MixedBulkWriteOperation.java:95)
at com.mongodb.MongoCollectionImpl.insertMany(MongoCollectionImpl.java:323)
at com.mongodb.MongoCollectionImpl.insertMany(MongoCollectionImpl.java:311)
at org.apache.beam.sdk.io.mongodb.MongoDbIO$Write$WriteFn.flush(MongoDbIO.java:513)
at org.apache.beam.sdk.io.mongodb.MongoDbIO$Write$WriteFn.finishBundle(MongoDbIO.java:506)