ROB: /AcroForm might be NullObject

[joshkersey]

This PR prevents PdfWriter.merge()/append() from crashing when a source PDF’s catalog contains an /AcroForm entry that is present but null (or resolves to null). A regression test is included.

Closes #3598

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.35%. Comparing base (26fd638) to head (3a84a66).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3601   +/-   ##
=======================================
  Coverage   97.35%   97.35%           
=======================================
  Files          55       55           
  Lines        9816     9816           
  Branches     1792     1792           
=======================================
  Hits         9556     9556           
  Misses        153      153           
  Partials      107      107           

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[stefan6419846]

Thanks.