Handle unsetting policy tags in BigQuery table schema

[modular-magician]

Fixes #14047.

Now all of 1) no policyTags, 2) policyTags = {}, and 3) policyTags = { names = [] } in schema will remove any existing policyTags setting on the designated table column, and no longer generate a permadiff after the removal is applied.

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Release Note Template for Downstream PRs (will be copied)

bigquery: added support to unset policy tags in table schema

Derived from GoogleCloudPlatform/magic-modules#8658

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