[Fix] Fix invalid hOCR format & PDF/A compatiblity with the kie preditor

[felixdittrich92]

This PR:

• Fix invalid hOCR format
• Fix xml export for kie_predictor
• Extend xml output test to check that the output is well-formed hOCR format

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.58%. Comparing base (3e21320) to head (dee867b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1870      +/-   ##
==========================================
- Coverage   96.63%   96.58%   -0.05%     
==========================================
  Files         166      166              
  Lines        7987     7990       +3     
==========================================
- Hits         7718     7717       -1     
- Misses        269      273       +4     

Flag	Coverage Δ
unittests	96.58% <100.00%> (-0.05%)	⬇️

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