Fixes #9602.

This PR fixes the issue when inserting rows with missing REPEATED fields. Omitting a REPEATED field from the request does not result in an error anymore.

How to test

Best to run the ready-made script from one of the issue comments.

MIND:
Omitting None values only happens when the code needs to convert the data to JSON by itself, i.e. in insert_rows(). The following thus still fails when insert_rows_json() is used:

rows = [{"int_col": 9, "repeated_col": None}
errors = client.insert_rows_json(TABLE_PATH, rows)

If this inconsistency is not desired, we need to sync the logic in these two methods.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)