Fix: Null key reference in bigquery cursor query results #53127

[baaaqt]

This update modifies the _format_schema_for_description function to check for the presence of the "fields" key in the schema before attempting to iterate over it.

closes: #53127

[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.