Fix issues with nullable properties in Bokeh 2.3

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #1948 (65688ff) into master (035c81b) will decrease coverage by 0.00%.
The diff coverage is 97.67%.

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
- Coverage   85.21%   85.20%   -0.01%     
==========================================
  Files         152      152              
  Lines       18981    18963      -18     
==========================================
- Hits        16175    16158      -17     
+ Misses       2806     2805       -1     

Impacted Files	Coverage Δ
panel/layout/tabs.py	87.50% <ø> (ø)
panel/pane/plotly.py	89.14% <ø> (ø)
panel/widgets/tables.py	84.60% <ø> (-0.03%)	⬇️
setup.py	0.00% <ø> (ø)
panel/widgets/select.py	93.51% <66.66%> (+0.28%)	⬆️
panel/models/layout.py	100.00% <100.00%> (ø)
panel/models/markup.py	100.00% <100.00%> (ø)
panel/models/tabulator.py	100.00% <100.00%> (ø)
panel/models/vega.py	92.85% <100.00%> (ø)
panel/models/vtk.py	98.43% <100.00%> (ø)
... and 7 more

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