Allow constructing empty HoloViews pane

[philippjfr]

Other Panes already allow an empty constructor but it seems like I overlooked HoloViews.

[codecov-io]

Codecov Report

Merging #307 into master will increase coverage by 0.19%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   89.07%   89.27%   +0.19%     
==========================================
  Files          63       63              
  Lines        6565     6583      +18     
==========================================
+ Hits         5848     5877      +29     
+ Misses        717      706      -11

Impacted Files	Coverage Δ
panel/tests/test_holoviews.py	99.21% <100%> (+0.03%)	⬆️
panel/pane/holoviews.py	83.88% <94.11%> (+0.55%)	⬆️
panel/pane/image.py	82.97% <0%> (+1.06%)	⬆️
panel/util.py	70.25% <0%> (+1.89%)	⬆️
panel/interact.py	75.09% <0%> (+2.64%)	⬆️

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