Allow Viewer to render servable but non-viewable objects

[philippjfr]

Fixes #4101

[codecov]

Codecov Report

Merging #4131 (52d75b8) into master (1be8a1a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4131   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files         224      224           
  Lines       33373    33373           
=======================================
  Hits        27898    27898           
  Misses       5475     5475           

Flag	Coverage Δ
ui-tests	37.62% <100.00%> (ø)
unitexamples-tests	74.59% <100.00%> (ø)

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Impacted Files	Coverage Δ
panel/pane/base.py	88.75% <100.00%> (ø)
panel/tests/util.py	89.32% <0.00%> (ø)

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