fix: RowSpan should work with Excel Export and merge cells

[ghiscoding]

also improve Text Export (csv/txt) but technically speaking the only thing we do in that export type is to change cell to empty string when a span is found because a we merge any cells in csv so the best we can do is to empty the cells... in other words, Excel Export has a much better UX because we do have access cell merge in Excel

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.5%. Comparing base (70e7222) to head (3fe0c08).
Report is 18 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1819     +/-   ##
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+ Coverage    99.5%   99.5%   +0.1%     
========================================
  Files         187     187             
  Lines       34442   34467     +25     
  Branches    10191   10202     +11     
========================================
+ Hits        34259   34284     +25     
  Misses        183     183             

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