Fix most access to unset attributes in assign_wcs

[emolter]

Resolves JP-nnnn

This PR addresses ...

Tasks

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  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
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    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.31%. Comparing base (bbf1e5f) to head (faa0b00).
⚠️ Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
jwst/lib/exposure_types.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10177      +/-   ##
==========================================
+ Coverage   85.98%   86.31%   +0.33%     
==========================================
  Files         368      366       -2     
  Lines       38457    38410      -47     
==========================================
+ Hits        33066    33155      +89     
+ Misses       5391     5255     -136     

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