On double-checking the various Cody Gateway access management CRUD I noticed that I did not implement the ability to remove the override entirely properly - right now, removing a rate limit with field_mask and no values results in a zero-value rate limit rather than a null one as desired.

This change makes it so that if you provider a nil *_rate_limit object, all field paths for that object is also set to nil, removing the override and allowing the default active-license-plan-based rate limits to apply.

This also adds a minor adjustment to Cody Gateway to correctly handle zero durations for rate limit intervals, which should be treated as "no access".

Test plan

Unit/integration tests, and a manual test:

• Check out https://github.com/sourcegraph/sourcegraph/pull/64090 and run sg start dotcom
• Go to a subscription, add a rate limit override
  
• Click the 🗑️ button to remove the rate limit override
  
• Repeat above on each property