Make Plotly.viewport nullable

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #1988 (7e66fa2) into master (3822f49) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1988   +/-   ##
=======================================
  Coverage   84.65%   84.65%           
=======================================
  Files         163      163           
  Lines       19723    19723           
=======================================
  Hits        16696    16696           
  Misses       3027     3027           

Impacted Files	Coverage Δ
panel/models/plotly.py	96.15% <100.00%> (ø)

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