test: parametrize to_arrow empty stream tests and update docstrings

[chalmerlowe]

Problem

When reading from an empty BigQuery table using the Read API, the stream may return no messages. Previously, this could lead to an AttributeError when trying to parse the schema from a None parser. While a fallback was added to return an empty table, this fallback uses an empty schema (zero columns), which can cause downstream consumers (like Vertex Ray) to fail if they expect specific columns to be present.

Solution

1. Updated Documentation: Added explanatory comments and updated docstrings in reader.py to clarify the flow between ReadRowsStream.to_arrow() and ReadRowsIterable.to_arrow(), and to inform users about the fallback behavior.
2. Added Parametrized Tests: Added comprehensive tests to test_reader_v1_arrow.py to verify the behavior of to_arrow() on empty streams, both with and without a read_session provided.

Notes to Reviewers

• This PR does not change functionality but adds testing and documentation around existing behavior to prevent future regression and aid debugging. The functionality that should ensure the correct behavior in internal bug #352600521 was added in February in PR # 15575. This PR is less about fixing the bug versus documenting the correct use of the code to avoid issues.
• To guarantee the correct schema (non-empty) for empty streams, callers should provide the read_session to ReadRowsStream.to_arrow().

Fixes internal bug #352600521

[gemini-code-assist]

Code Review

This pull request adds documentation and inline comments to clarify the behavior of to_arrow() on empty streams in ReadRowsIterable and ReadRowsStream. It also introduces a new unit test, test_to_arrow_empty_stream, to verify empty stream handling. Feedback on the unit test highlights a potential TypeError when testing ReadRowsIterable because it does not accept a read_session argument, and suggests a code modification to conditionally handle this case.