Fix issue when validating an OpenAPI 3.1.0 definition

[AnuGayan]

Fixes wso2/api-manager#2661

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.83%. Comparing base (e94b4fd) to head (0d119ac).
Report is 19 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12346      +/-   ##
============================================
+ Coverage     47.62%   47.83%   +0.21%     
- Complexity     4192     4211      +19     
============================================
  Files          1840     1840              
  Lines        136532   136532              
  Branches      19767    19767              
============================================
+ Hits          65019    65308     +289     
+ Misses        63857    63548     -309     
- Partials       7656     7676      +20     

Flag	Coverage Δ
integration_tests	42.70% <100.00%> (+0.27%)	⬆️
unit_tests	20.17% <100.00%> (ø)

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