Tolerate AcroForms without a Fields entry

[MatthiasValvekens]

Description of the changes

Tolerate AcroForm dictionaries that don't contain a Fields entry when making changes to a PDF. We'll create it on the fly if needed, even though the input document would strictly speaking not be valid.

Fixes #403.

Checklist

Please go over this checklist to increase the chances of your PR being worked on in a timely manner. Deviations are allowed with proper justification (see previous section).

• I have read the project's CoC and contribution guidelines.
• I understand and agree to the terms in the Developer Certificate of Origin as applied to this contribution.
• All new code in this PR has full test coverage.

For bug fixes (delete if not applicable)

• My changes do not affect any public API or CLI semantics.
• My PR adds regression tests (i.e. tests that fail if the bug fix is not applied).
• All new code in this PR has full test coverage.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.83%. Comparing base (5a9d8eb) to head (6d4cde9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #404   +/-   ##
=======================================
  Coverage   98.83%   98.83%           
=======================================
  Files         113      113           
  Lines       16265    16265           
=======================================
  Hits        16075    16075           
  Misses        190      190           

Flag	Coverage Δ
unittests	98.83% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.