Add null check to Json.Get<T>

[shacharPash]

Closes #198

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e32b6b6) 94.05% compared to head (839fce1) 94.12%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   94.05%   94.12%   +0.07%     
==========================================
  Files          85       85              
  Lines        5178     5178              
  Branches      488      488              
==========================================
+ Hits         4870     4874       +4     
+ Misses        181      177       -4     
  Partials      127      127              

Files	Coverage Δ
src/NRedisStack/Json/JsonCommands.cs	89.65% <0.00%> (+1.37%)	⬆️
src/NRedisStack/Json/JsonCommandsAsync.cs	89.65% <0.00%> (+1.37%)	⬆️

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[slorello89]

LGTM 👍