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).

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%)	⬆️

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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?