Fixed bugs with None default on String params

[philippjfr]

Fixes #1598

[philippjfr]

@mattpap What should the property type be for a property that can be either null or a p.String?

[philippjfr]

Nevermind, found the NullString type.

[codecov]

Codecov Report

Merging #1622 into master will increase coverage by 0.10%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1622      +/-   ##
==========================================
+ Coverage   85.26%   85.36%   +0.10%     
==========================================
  Files         147      147              
  Lines       16663    16779     +116     
==========================================
+ Hits        14207    14323     +116     
  Misses       2456     2456              

Impacted Files	Coverage Δ
panel/widgets/tables.py	89.57% <50.00%> (-0.38%)	⬇️
panel/io/model.py	60.63% <0.00%> (-0.34%)	⬇️
panel/tests/test_server.py	100.00% <0.00%> (ø)
panel/tests/test_template.py	97.91% <0.00%> (+0.32%)	⬆️
panel/viewable.py	70.95% <0.00%> (+0.44%)	⬆️
panel/io/server.py	68.57% <0.00%> (+2.44%)	⬆️
panel/io/state.py	48.87% <0.00%> (+3.37%)	⬆️
panel/io/location.py	85.56% <0.00%> (+4.12%)	⬆️

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