Fix handling of string dtypes for Perspective

[philippjfr]

Partial fix for #5081

[codecov]

Codecov Report

Merging #5467 (86dd134) into main (92e3982) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##             main    #5467   +/-   ##
=======================================
  Coverage   72.57%   72.57%           
=======================================
  Files         274      275    +1     
  Lines       40111    40124   +13     
=======================================
+ Hits        29109    29121   +12     
- Misses      11002    11003    +1     

Flag	Coverage Δ
unitexamples-tests	72.57% <80.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
panel/pane/perspective.py	58.86% <80.00%> (+1.50%)	⬆️

... and 5 files with indirect coverage changes

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