[c++] Fix bug in nnz of variant-indexed dataframes

[johnkerl]

Issue and/or context: Pre-existing bug found while working on issue #2407 / [sc-51048].

I ran sdf.count() on a variant-indexed dataframe which has dim0 being a string (which the spec allows even though most users don't take advantage of this). I got the error

[TileDB::FragmentInfo] Error: Cannot get non-empty domain; Dimension is variable-sized

There is a repro now built into the unit-test cases on this PR.

Changes:

Only use the code that assumes dim0 is int64 soma_joinid when that is in fact the case.

Notes for Reviewer:

[johnkerl]

Leaving in draft while I address a unit-test failure

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (4a13a82) to head (265c9b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2990      +/-   ##
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+ Coverage   89.64%   89.79%   +0.14%     
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  Files          40       40              
  Lines        4096     4096              
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+ Hits         3672     3678       +6     
+ Misses        424      418       -6     

Flag	Coverage Δ
python	89.79% <ø> (+0.14%)	⬆️

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Components	Coverage Δ
python_api	89.79% <ø> (+0.14%)	⬆️
libtiledbsoma	∅ <ø> (∅)