Explicitly handle if the discriminator property value is null

[justin-tay]

Closes #985

This makes changes such that a missing discriminator property is treated as if the discriminator is ignored.

This does not generate an assertion due to the missing discriminator property to be consistent with

• Relaxation of the discriminator validation #436

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.81%. Comparing base (0f983b0) to head (be2d8f9).

Files	Patch %	Lines
...main/java/com/networknt/schema/AnyOfValidator.java	80.00%	0 Missing and 1 partial ⚠️
...main/java/com/networknt/schema/OneOfValidator.java	94.73%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #988      +/-   ##
============================================
+ Coverage     78.76%   78.81%   +0.05%     
- Complexity     1919     1927       +8     
============================================
  Files           172      172              
  Lines          6197     6213      +16     
  Branches       1199     1205       +6     
============================================
+ Hits           4881     4897      +16     
  Misses          861      861              
  Partials        455      455              

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