Set default for description to None

[hoxbro]

Fixes #4617

[philippjfr]

Great, thanks @hoxbro. Could you maybe create an issue with a table that summarizes which widget do and do not support tooltips and link it with the equivalent issue for widget titles (which I am having trouble finding right now)?

[codecov]

Codecov Report

Merging #4621 (5d85e84) into main (e179a2d) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4621      +/-   ##
==========================================
- Coverage   73.32%   73.31%   -0.02%     
==========================================
  Files         260      260              
  Lines       37127    37127              
==========================================
- Hits        27224    27219       -5     
- Misses       9903     9908       +5     

Flag	Coverage Δ
unitexamples-tests	73.31% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/input.py	95.16% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[philippjfr]

Thanks @hoxbro, looks good.