DECam instcals fail to process with invalid DateTime

When a user ingests a DECam instcal and runs exposure.getVisitInfo().getDate(), it returns an empty DateTime. ProcessCcd naturally assumes a non-empty DateTime and fails with a "DateTime not valid" error.

We think this problem originates in astro_metadata_translator, which likely was made to work with DECam raws and not DECam instcals. Instcals are apparently missing some of the expected header keywords.

This ticket is to fix things so ingested instcals have proper DateTimes and to add a test (presumably in astro_metadata_translator) that checks an instcal header.

See the full discussion on Slack here.