Various fixes to allow formatting and editing null date values

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #3008 (6b8510b) into master (c91dd57) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3008      +/-   ##
==========================================
- Coverage   83.25%   83.24%   -0.01%     
==========================================
  Files         190      190              
  Lines       25024    25024              
==========================================
- Hits        20834    20832       -2     
- Misses       4190     4192       +2     

Impacted Files	Coverage Δ
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️

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