Fix handling of single and empty options on DiscreteSlider

[philippjfr]

Fixes #1804
Fixes #2394
Fixes #2395

[codecov]

Codecov Report

Merging #3297 (03b5e15) into master (7810c1e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3297      +/-   ##
==========================================
+ Coverage   83.38%   83.41%   +0.03%     
==========================================
  Files         195      195              
  Lines       26387    26423      +36     
==========================================
+ Hits        22003    22041      +38     
+ Misses       4384     4382       -2     

Impacted Files	Coverage Δ
panel/tests/widgets/test_slider.py	100.00% <100.00%> (ø)
panel/widgets/slider.py	84.85% <100.00%> (+0.69%)	⬆️

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