Multiordered combined sky maps failing with Burst GW sky maps

The method to create a combined sky map involving a Burst GW sky map appear to fail due to the lack of a DISTMU field: https://ligo-caltech.sentry.io/issues/4041604730/?query=is%3Aunresolved&referrer=issue-stream&stream_index=12

This could be fixed by doing a check of the DISTMU and related fields in order to update the distances in the combined sky map: https://git.ligo.org/emfollow/gwcelery/-/blob/3c4272f9968ecc8e4e20dcaaf1280bbaca81d013/gwcelery/tasks/external_skymaps.py#L165-168