As seen in internal bug 182204971, #557, if the policyTags key is included in a load job, the user can get 403 POST ... User does not have permission to get taxonomy ... errors when uploading data, even if they have write access to the table.

Based on the behavior seen in googleapis/python-bigquery-pandas#387, I believe this fix has accidentally been reverted in #703 (comment).

TODO:

• Manually test to see if the error from 182204971 is reproduced
• Add a system test to avoid future regressions
• Update SchemaField to allow disambiguating unset vs explicitly set to empty list / none.