Fix regression handling selection on multi-indexed Tabulator data

[philippjfr]

Fixes regression introduced in #7304

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.21%. Comparing base (d39874c) to head (edeaa0a).
Report is 3 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%   82.21%   -0.01%     
==========================================
  Files         337      337              
  Lines       50503    50513      +10     
==========================================
+ Hits        41521    41527       +6     
- Misses       8982     8986       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.