Fix Cassini Mission phase NULL problem

[rfrenchseti]

• Fixes #N/A
• Were any Django, import pipeline, or table_schema files modified? Y
• Were any JavaScript or CSS files modified? N
  • If YES:
  • JSHINT run on all affected files: Y/NA
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y/NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): Y/NA
• Was the documentation reviewed for necessary changes or additions? NA
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Description of changes:

Some Cassini ISS observations have a MISSION_PHASE_NAME of "NULL" and we were converting these to a mult of None, which isn't permitted. Now we infer the proper mission phase name from the observation time in these cases.

Known problems:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (62d7c27) to head (7aa6a5c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1377 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 41 41 Lines 25084 25084 Branches 1221 1221 ========================================= Hits 25084 25084 ```

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