add test that ApcorrRefModel arrays can be None

[braingram]

This PR adds a test that checks if ApcorrRefModel arrays can be None.

The test fails with rad main and all versions of asdf due to a bug in the datatype validator.

spacetelescope/rad#542 works around the issue until an upstream fix and release can be made by reordering the None check in the anyOf combiner. The test added in this PR will provide some safeguard against reintroduction of this bug.

Tasks

• Update or add relevant roman_datamodels tests.
• Update relevant docstrings and / or docs/ page.
• 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>").

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 97.25%. Comparing base (087a60d) to head (5950aaa).
Report is 105 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
- Coverage   97.56%   97.25%   -0.31%     
==========================================
  Files          30       37       +7     
  Lines        2788     3315     +527     
==========================================
+ Hits         2720     3224     +504     
- Misses         68       91      +23     

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[schlafly]

Looks good to me, thanks!