Do not calculate embed state for disabled widgets

[philippjfr]

Fixes #1295

[codecov]

Codecov Report

Merging #3757 (babc310) into master (4a9b4d0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3757      +/-   ##
==========================================
+ Coverage   84.21%   84.23%   +0.01%     
==========================================
  Files         213      213              
  Lines       30993    31002       +9     
==========================================
+ Hits        26102    26113      +11     
+ Misses       4891     4889       -2     

Flag	Coverage Δ
ui-tests	33.99% <10.00%> (-0.01%)	⬇️
unitexamples-tests	77.13% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/io/embed.py	86.87% <100.00%> (ø)
panel/tests/io/test_embed.py	99.42% <100.00%> (+0.01%)	⬆️
panel/io/document.py	66.97% <0.00%> (+1.83%)	⬆️

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