Fix nullable issue

[justin-tay]

Closes #1133

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.09%. Comparing base (48ca3c2) to head (67fbd68).
Report is 74 commits behind head on master.

Files with missing lines	Patch %	Lines
.../java/com/networknt/schema/utils/JsonNodeUtil.java	0.00%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1134      +/-   ##
============================================
- Coverage     78.90%   78.09%   -0.81%     
- Complexity     1965     2058      +93     
============================================
  Files           172      201      +29     
  Lines          6352     6849     +497     
  Branches       1255     1251       -4     
============================================
+ Hits           5012     5349     +337     
- Misses          867     1033     +166     
+ Partials        473      467       -6     

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