Fix NPE when walking a missing node that will have missing properties

[justin-tay]

Closes #1151

Fixes the NPE by having TypeFactory.getValueNodeType((JsonNode node, SchemaValidatorsConfig config) handle the case where a null was passed in as the node.

I had considered if it should be ensured that the node inputs should always be non null and always replace null by MissingNode instead. For instance having PropertiesValidator set the input as MissingNode instead of null. But since JsonNode.get(String fieldName) returns null for missing nodes I wasn't sure it would be appropriate.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.13%. Comparing base (48ca3c2) to head (5ae2fff).
⚠️ Report is 97 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1152      +/-   ##
============================================
- Coverage     78.90%   78.13%   -0.77%     
- Complexity     1965     2066     +101     
============================================
  Files           172      201      +29     
  Lines          6352     6861     +509     
  Branches       1255     1255              
============================================
+ Hits           5012     5361     +349     
- Misses          867     1033     +166     
+ Partials        473      467       -6     

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[jkosternl]

Just 2 minor remarks, to take into consideration. Good job!

[stevehu]

@justin-tay The latest commit is not included in the previous PR which is merged so quickly. Could you please submit another PR to include the test case? Thanks.

[justin-tay]

@stevehu It looks like the latest commit to add the assertDoesNotThrow was included in the merged commit da3865c so there's nothing more. Thanks!