BUG: Do not fail on choice field without /Opt key

[jhuber-de]

This PR fixes a issue (#2838) with reading of choice fields that are missing the optinal /Opt key.

Note: I added a unittest with a new example pdf file. I'm not sure if the file should be added in this repo, in the sample-files repo or if it should be downloaded on demand with get_data_from_url() from the original source.
I have not committed the original file with the issue since, while technically publicly available, I have not created that PDF file myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.16%. Comparing base (310e571) to head (4710d78).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3540      +/-   ##
==========================================
- Coverage   97.16%   97.16%   -0.01%     
==========================================
  Files          57       57              
  Lines        9809     9808       -1     
  Branches     1781     1781              
==========================================
- Hits         9531     9530       -1     
  Misses        167      167              
  Partials      111      111              

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[stefan6419846]

I'm not sure if the file should be added in this repo, in the sample-files repo or if it should be downloaded on demand with get_data_from_url() from the original source.

For these cases, we regularly use a local copy downloaded from a GitHub comment link as we cannot safely ship the file with pypdf directly in these cases.

Feel free to use the following link: Musterservicevertrag-HNRAGB_Okt2022-Blanko.pdf

[stefan6419846]

Thanks.