Ensure param callbacks initialized

[philippjfr]

Fixes #1327

[codecov]

Codecov Report

Merging #1439 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1439      +/-   ##
==========================================
+ Coverage   87.90%   87.92%   +0.01%     
==========================================
  Files         135      135              
  Lines       14879    14993     +114     
==========================================
+ Hits        13079    13182     +103     
- Misses       1800     1811      +11     

Impacted Files	Coverage Δ
panel/param.py	91.59% <100.00%> (+0.03%)	⬆️
panel/tests/test_param.py	99.86% <100.00%> (+<0.01%)	⬆️
panel/viewable.py	70.17% <0.00%> (+0.39%)	⬆️
panel/io/server.py	71.80% <0.00%> (+9.04%)	⬆️

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