Is your feature request related to a problem? Please describe.

I noticed in#557 that we don't send the policy tags property if none are set in SchemaField. I believe this tells the backend not to make any updates to the policy tags property. This means it's not possible tounset policy tags from Python.

(Note: I haven't verified this, so some system tests / samples should be written for this use case)

Describe the solution you'd like

Similar to what I did fordescription in#557, we should disambiguate between unset policy tags and empty / none for policy tags.

Describe alternatives you've considered

We could make the update fields property more fine-grained, such asschema.policytags in order to force a sub-property to be included. This feels overly complicated, though.

Additional context

N/A