Fix 'RuntimeValueProvider' object has no attribute 'projectId' error in _CustomBigQuerySource.split

Error message from worker: Traceback (most recent call last): 
File "/usr/local/lib/python3.7/site-packages/dataflow_worker/batchworker.py", line 644, 
in do_work work_executor.execute() File "/usr/local/lib/python3.7/site-packages/dataflow_worker/executor.py", line 255, 
in execute self._split_task) File "/usr/local/lib/python3.7/site-packages/dataflow_worker/executor.py", line 263, 
in _perform_source_split_considering_api_limits desired_bundle_size) File "/usr/local/lib/python3.7/site-packages/dataflow_worker/executor.py", line 300, 
in _perform_source_split for split in source.split(desired_bundle_size): File "/usr/local/lib/python3.7/site-packages/apache_beam/io/gcp/bigquery.py", line 813, 
in split if not self.table_reference.projectId: AttributeError: 'RuntimeValueProvider' object has no attribute 'projectId'