Don't update header in combined sky map if missing in GW sky map; fixes #576

This MR now checks that various fields in the GW sky map are present before trying to update them. This should fix [this Sentry error](

• https://ligo-caltech.sentry.io/issues/4041604730/?query=is%3Aunresolved&referrer=issue-stream&stream_index=12) as well as any future oversights in GW sky maps.