Fix walk for if validator with validation

[justin-tay]

Closes #1009

Previously with the validate schema option enabled, the walk would delegate to validate. But calling validate on the schemas will not walk the schema.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.90%. Comparing base (fbf11c3) to head (7759a24).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...rc/main/java/com/networknt/schema/IfValidator.java	73.33%	0 Missing and 4 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1010      +/-   ##
============================================
+ Coverage     78.80%   78.90%   +0.10%     
- Complexity     1960     1965       +5     
============================================
  Files           172      172              
  Lines          6345     6352       +7     
  Branches       1253     1255       +2     
============================================
+ Hits           5000     5012      +12     
+ Misses          876      867       -9     
- Partials        469      473       +4     

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