Fixing validation of advance key check in EN16931 addon

[samlown]

• Fixes issue when validating presence of a payment advance key, which is not a strict requirement of EN16931
• Also including fix to reduce requirements on payment card fields

Pre-Review Checklist

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents to show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes, either structured or in the comments.
• I've run go generate . to ensure that the Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.15%. Comparing base (191492f) to head (49c964e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   85.17%   85.15%   -0.03%     
==========================================
  Files         255      255              
  Lines       12595    12576      -19     
==========================================
- Hits        10728    10709      -19     
  Misses       1518     1518              
  Partials      349      349              

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