Reset DataFrame.RowCount to zero, when DataFrame is empty

[asmirnov82]

Reset DataFrame.RowCount to zero after Columns collection was cleared or the last column was removed.
Add addtional unit tests on Adding, Inserting and Removing columns in DataFrame

Fixes #6694

[codecov]

Codecov Report

Merging #6698 (8e07da9) into main (d18676b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6698   +/-   ##
=======================================
  Coverage   68.84%   68.85%           
=======================================
  Files        1215     1215           
  Lines      250691   250725   +34     
  Branches    26256    26257    +1     
=======================================
+ Hits       172593   172631   +38     
+ Misses      71278    71271    -7     
- Partials     6820     6823    +3     

Flag	Coverage Δ
Debug	68.85% <100.00%> (+<0.01%)	⬆️
production	63.35% <100.00%> (+<0.01%)	⬆️
test	88.88% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...crosoft.Data.Analysis/DataFrameColumnCollection.cs	81.93% <100.00%> (+2.11%)	⬆️
...st/Microsoft.Data.Analysis.Tests/DataFrameTests.cs	99.25% <100.00%> (+<0.01%)	⬆️

... and 10 files with indirect coverage changes

[michaelgsharp]

LGTM. Thanks for all your hard work in DataFrames.

[JakeRadMSFT]

Merged into Generic Math branch. (This is just a note for myself)