Add handling of incomplete dynamic extension data

[blink1073]

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[codecov-commenter]

Codecov Report

Merging #124 into master will increase coverage by 0.20%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   58.96%   59.17%   +0.20%     
==========================================
  Files          20       20              
  Lines        1572     1575       +3     
==========================================
+ Hits          927      932       +5     
+ Misses        645      643       -2     

Impacted Files	Coverage Δ
jupyterlab_server/config.py	24.06% <0.00%> (-0.56%)	⬇️
jupyterlab_server/tests/utils.py	28.26% <0.00%> (+10.86%)	⬆️

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