FIX: ensure required_tag_group_name is null if no value present (PR #14796)

If the array was present but empty required_tag_group_name would be set to undefined, which would then be removed from the payload of the remote request.

Addming the length check ensures the value is set to null, which is sent as an empty value (which the backend sees, and can remove it and persist the change on the Category object).