Handle zero sky map overlap case gracefully

We occasionally get errors where there is no sky map overlap between two sky maps, usually because of using very well localized Swift events.

We should think about the best way to handle these errors, potentially doing one of the following:

• If a result of zero is seen upstream, a more informative error message is produced and the system does not try to update the superevent with this new info.
• A zero is remapped to some other arbitrarily small value and allowed to update the superevent.

See this Sentry Issue: https://ligo-caltech.sentry.io/issues/6607381618/?environment=playground&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&stream_index=3