Switch Param pane widget type when bounds (un)defined

[philippjfr]

Fixes #1951

[codecov]

Codecov Report

Merging #1953 (092ba46) into master (588f074) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1953      +/-   ##
==========================================
+ Coverage   85.19%   85.22%   +0.03%     
==========================================
  Files         154      154              
  Lines       18987    19019      +32     
==========================================
+ Hits        16176    16209      +33     
+ Misses       2811     2810       -1     

Impacted Files	Coverage Δ
panel/param.py	90.90% <100.00%> (+0.56%)	⬆️
panel/tests/test_param.py	99.87% <100.00%> (+<0.01%)	⬆️

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