Fix walk with validate when node is null

[justin-tay]

Closes #1167

Should not actually validate when walking with node that is null.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.26%. Comparing base (48ca3c2) to head (b2aad39).
Report is 89 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/java/com/networknt/schema/NotValidator.java	50.00%	0 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/OneOfValidator.java	50.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1169      +/-   ##
============================================
- Coverage     78.90%   78.26%   -0.65%     
- Complexity     1965     2070     +105     
============================================
  Files           172      201      +29     
  Lines          6352     6855     +503     
  Branches       1255     1254       -1     
============================================
+ Hits           5012     5365     +353     
- Misses          867     1019     +152     
+ Partials        473      471       -2     

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