Fix errors with producing alerts for SNEWS coincidences

After introducing MDC SNEWS events, there have been several errors that have appeared in Sentry when coincidences were randomly created with Burst events:

external_skymaps.plot_overlap_integral failing when passed an empty coinc FAR dict: https://ligo-caltech.sentry.io/issues/4134391007/?query=is%3Aunresolved&referrer=issue-stream&stream_index=13
❌ If group gets switched back to CBC, we should allow GRBs to be able to be chosen as the preferred external event
Fix circulars breaking for SNEWS coincidences: https://ligo-caltech.sentry.io/issues/4134391007/?query=is%3Aunresolved&referrer=issue-stream&stream_index=13

Edited May 03, 2023 by Brandon Piotrzkowski

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