RAD-189: APCORR and ABVEGAOFFSET optional nulls

[PaulHuwe]

Resolves RAD-189

Closes #488

This PR adds null values to allowed APCORR and ABVEGAOFFSET keyword values.

Tasks

• Update or add relevant rad tests.
• Update relevant docstrings and / or docs/ page.
• Does this PR change any schema files?
  • Schema changes were discussed at RAD Review Board meeting.
• Does this PR change any API used downstream? (If not, label with no-changelog-entry-needed.)
  • Write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types).
  • Start a romancal regression test (https://github.com/spacetelescope/RegressionTests/actions/workflows/romancal.yml) with this branch installed ("git+https://github.com/<fork>/rad@<branch>").
  • Update relevant roman_datamodels utilities and tests.

News fragment change types:

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.81%. Comparing base (f92cfab) to head (a940e46).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files           4        4           
  Lines         215      215           
=======================================
  Hits          206      206           
  Misses          9        9           

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[schlafly]

Looks good to me, but let's see if this meets Will's needs.

[PaulHuwe]

@wcschultz Can you confirm if this if it is what you need?

[wcschultz]

The changes look good to me. Thanks!