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

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'

```

Imported from Jira [BEAM-14478](https://issues.apache.org/jira/browse/BEAM-14478). Original Jira may contain additional context.
Reported by: heejong.