Remove check for at least one schema in GCSToBigquery

[c-nuro]

For the case when updating an existing table or insert data to a particular partition, no schema is needed.
Autodetect doesn't always work, e.g. cannot distinguish partition correctly. Other options requires forking the schema to airflow.

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[boring-cyborg]

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[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[turbaszek]

@c-nuro can you please fix the static checks?

airflow/providers/google/cloud/transfers/gcs_to_bigquery.py:23:1: F401 'airflow.exceptions.AirflowException' imported but unused

[boring-cyborg]

Awesome work, congrats on your first merged pull request!