6 arcs causes psfnight to be skipped

[sbailey]

Night 20231202 had 6 short arcs instead of the standard 5. This confused the pipeline logic, causing the psfnight to not be submitted and the flats to be run without waiting for the psfs.

@akremin is rewriting the pipeline logic, but we should explicitly test and be robust to cases like this, with more than expected good exposures.