hocr: only add space if boxwidth is positive

[pajowu]

If the HOCR contains overlapping words in the same line or the words are not ordered in reading-direction, space_box.width might be negative. This leads to more issues with the calculation of font width etc, producing text boxes that are way larger than the actual bbox set within the hocr-file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.20%. Comparing base (02d85ff) to head (74253e5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1446      +/-   ##
==========================================
+ Coverage   90.18%   90.20%   +0.01%     
==========================================
  Files          95       95              
  Lines        7073     7073              
  Branches      722      722              
==========================================
+ Hits         6379     6380       +1     
  Misses        491      491              
+ Partials      203      202       -1     

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[jbarlow83]

This is great work - I just want to take some time to review it.