Handle cancelling and empty value edit events on Tabulator

[philippjfr]

• Adds validators on Numeric edit events
• Ensures that cancelling edit event does not clear cell
• Maps an empty cell numeric cell to NaN

Fixes #3774

[codecov]

Codecov Report

Merging #4343 (34a392e) into main (56a64cb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4343      +/-   ##
==========================================
+ Coverage   82.77%   82.78%   +0.01%     
==========================================
  Files         240      240              
  Lines       34599    34631      +32     
==========================================
+ Hits        28639    28669      +30     
- Misses       5960     5962       +2     

Flag	Coverage Δ
ui-tests	38.33% <100.00%> (+0.05%)	⬆️
unitexamples-tests	73.75% <9.37%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
panel/tests/ui/widgets/test_tabulator.py	99.11% <100.00%> (+0.11%)	⬆️
panel/tests/util.py	89.32% <0.00%> (-1.95%)	⬇️
panel/io/convert.py	71.77% <0.00%> (-0.36%)	⬇️

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