Fix regression handling selection on multi-indexed Tabulator data

[philippjfr]

Fixes regression introduced in #7304

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.20%. Comparing base (d39874c) to head (a0d4009).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/widgets/test_tabulator.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7336      +/-   ##
==========================================
- Coverage   82.21%   82.20%   -0.01%     
==========================================
  Files         337      337              
  Lines       50503    50516      +13     
==========================================
+ Hits        41521    41529       +8     
- Misses       8982     8987       +5     

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