Ensure embedded slider initializes with correct default

[philippjfr]

Fixes #4106

[codecov]

Codecov Report

Merging #4121 (2540bc3) into master (0354e10) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4121   +/-   ##
=======================================
  Coverage   83.57%   83.57%           
=======================================
  Files         223      223           
  Lines       33280    33294   +14     
=======================================
+ Hits        27815    27827   +12     
- Misses       5465     5467    +2     

Flag	Coverage Δ
ui-tests	37.59% <6.66%> (-0.02%)	⬇️
unitexamples-tests	74.62% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/tests/io/test_embed.py	99.44% <100.00%> (+0.02%)	⬆️
panel/widgets/slider.py	95.14% <100.00%> (+<0.01%)	⬆️
panel/tests/util.py	89.32% <0.00%> (-1.95%)	⬇️

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