René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"add ability to unset policy tags (and description?) in schema fields","articleBody":" **Is your feature request related to a problem? Please describe.**\r\n\r\nI noticed in https://github.com/googleapis/python-bigquery/pull/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 to _unset_ policy tags from Python.\r\n\r\n(Note: I haven't verified this, so some system tests / samples should be written for this use case)\r\n\r\n **Describe the solution you'd like**\r\n\r\nSimilar to what I did for `description` in https://github.com/googleapis/python-bigquery/pull/557, we should disambiguate between unset policy tags and empty / none for policy tags.\r\n\r\n **Describe alternatives you've considered**\r\n\r\nWe could make the update fields property more fine-grained, such as `schema.policytags` in order to force a sub-property to be included. This feels overly complicated, though.\r\n\r\n **Additional context**\r\n\r\nN/A\r\n","author":{"url":"https://github.com/tswast","@type":"Person","name":"tswast"},"datePublished":"2021-03-18T15:23:55.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/558/python-bigquery/issues/558"}