ISIS version(s) affected: 7.0.0+

Description
The changes from #4589 broke ingesting cassini ISS data that has VALID_MAXIMUM=(UNK, UNK) in the original label. These data are largely earlier images taken of the Jupiter flyby and does not persist into the later data acquisition of Saturn and its moons.

It seems like if the VALID_MAXIMUM is unknown the validMax being set in the if/else logic before #4589 should persist. I have fixed the issue locally and plan to submit a PR.

Largely I am posting this issue for potential feedback from @michaelaye

How to reproduce
Grab this image and the associated label and try to ingest the data with ciss2isis

Possible Solution
Update the ingestion program to set a default validMax then check to make sure the PVL keyword VALID_MAXIMUM is not UNK

Additional context