Tabulator: fix the edit event with a python filter

[maximlt]

Fixes a bug that occurred while the edit event was being processed, when a Python filter was active. This bug led to the old value not being able to be set, as the _old data was then filtered but on the client the table is still the filtered one (i.e. it is not live filtered on edit, which is consistent with how Tabulator JS works and is the most practical approach).

This PR also returns early in _process_event when no edit/click callback is registered.

[codecov]

Codecov Report

Merging #3829 (c7f2efc) into master (87f79fa) will increase coverage by 7.51%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master    #3829      +/-   ##
==========================================
+ Coverage   75.48%   83.00%   +7.51%     
==========================================
  Files         221      221              
  Lines       32328    32367      +39     
==========================================
+ Hits        24402    26865    +2463     
+ Misses       7926     5502    -2424     

Flag	Coverage Δ
ui-tests	34.22% <82.35%> (?)
unitexamples-tests	75.39% <20.58%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/tables.py	88.41% <83.33%> (+0.96%)	⬆️
panel/reactive.py	78.31% <100.00%> (+0.74%)	⬆️
panel/tests/ui/widgets/test_tabulator.py	98.87% <100.00%> (+88.96%)	⬆️
panel/io/convert.py	28.62% <0.00%> (-0.89%)	⬇️
panel/pane/base.py	87.91% <0.00%> (+0.41%)	⬆️
panel/widgets/slider.py	95.14% <0.00%> (+0.74%)	⬆️
panel/util.py	87.16% <0.00%> (+0.75%)	⬆️
panel/tests/pane/test_vtk.py	100.00% <0.00%> (+1.48%)	⬆️
... and 18 more

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