Improve handling of missing `obs_end` value

As mentioned in issue #17

> We have discovered that sometimes the USDF Nightly Digest API at `/nightlydigest/api/exposures` will sometimes respond without an obs_end value set even though the observations period is well over.

As a temporary fix we implemented a [hard override to force the dome closed ](https://github.com/lsst-epo/nightly-digest-client-function/blob/cbf37f1996cd918d0723babae7a80209c3b30b1d/index.ts#L89-L99).

We should consider how to handle this in a less brutish way.

Start with index.ts at lines 89-99, then inspect how responses from /nightlydigest/api/exposures are handled when obs_end is absent. Determine the expected behavior for an observation period that has ended and define completion as replacing the temporary hard override with a robust handling path.

Written by the indexing model from the issue text.