Fix BigQueryCursor execute method if the location is missing

[e-galan]

This addresses a bug where BigQueryValueCheckOperator tasks failed if it was not provided with the location parameter and the dataset location was outside of the US.

Make BigQueryCursor use the job location in the case when the location is not provided during the cursor's instantiation.

Update typing and docstrings.

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[ahidalgob]

Hi, could you elaborate on how you tested this change?

[e-galan]

Hi, could you elaborate on how you tested this change?

@ahidalgob I ran a dag with a BigQueryValueCheckOperator task in a Composer environment that contained the code, with and without locations specified. With this code the BigQueryValueCheckOperator task finished successfully.

Adding new unit tests wasn't particularly useful because the execute() method essentially contained two client method calls (self._run_query(sql) and self._get_query_result()) that can only be mocked and not truly tested.

No existing unit tests have been broken / affected by this fix.