[BigqueryIO] MoreObjects.firstNonNull should not be called with null as default value

https://github.com/apache/beam/blob/2e0f79ef2b924da3306d0e55dffd4728809b9f3d/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/StorageApiWritesShardedRecords.java#L516 if streamName.read() returns null, firstNonNull will throw NullPointerException. We check the emptiness in the next line so it's ok for stream to be empty string, we should use empty string as the default value.