Fix bug that causes Merge to fail with empty DataFrames

[sevenzees]

Fixes #7572

When using the DataFrame Merge method, if one of the DataFrames is empty (has no rows), you will get a System.ArgumentOutOfRangeException. This was caused by the Clone method trying to access the first element of the empty Buffers array. It is easily fixed by returning an empty container of the appropriate length when Buffers.Count == 0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.07%. Comparing base (25b977e) to head (17caca4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7573      +/-   ##
==========================================
+ Coverage   69.02%   69.07%   +0.04%     
==========================================
  Files        1482     1483       +1     
  Lines      274099   274362     +263     
  Branches    28266    28270       +4     
==========================================
+ Hits       189199   189503     +304     
+ Misses      77518    77478      -40     
+ Partials     7382     7381       -1     

Flag	Coverage Δ
Debug	69.07% <100.00%> (+0.04%)	⬆️
production	63.33% <100.00%> (+0.01%)	⬆️
test	89.51% <100.00%> (+0.04%)	⬆️

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

Files with missing lines	Coverage Δ
...icrosoft.Data.Analysis/PrimitiveColumnContainer.cs	86.58% <100.00%> (+0.06%)	⬆️
...t/Microsoft.Data.Analysis.Tests/DataFrameAssert.cs	100.00% <100.00%> (ø)
...rosoft.Data.Analysis.Tests/DataFrameTests.Merge.cs	99.09% <100.00%> (+0.50%)	⬆️

... and 5 files with indirect coverage changes

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[sevenzees]

Looks like some builds timed out, which doesn't appear to be related to my commit. Is there some way to run them again?

[sevenzees]

@tarekgh I see that you have been active in this repo recently. Are you able to review this PR or point me to someone who can?

[tarekgh]

Thanks @sevenzees!

@ericstj the change LGTM. Please let me know if you have any feedback or we are good to merge. Thanks!

CC @jeffhandley

[ericstj]

cc @rokonec @ManickaP -- it looks like Mac CI is timing out. Might be something to look into -- seems like the helix jobs are sitting in queue and not running.