Fix oneOf when discriminator enabled but no discriminator

[justin-tay]

Fixes issue when oneOf when discriminator enabled but no discriminator causes no details in errors. Such cases should behave as a normal oneOf.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.98%. Comparing base (48ca3c2) to head (ebb2b65).
Report is 42 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1078      +/-   ##
============================================
- Coverage     78.90%   77.98%   -0.92%     
- Complexity     1965     2053      +88     
============================================
  Files           172      198      +26     
  Lines          6352     6855     +503     
  Branches       1255     1257       +2     
============================================
+ Hits           5012     5346     +334     
- Misses          867     1032     +165     
- Partials        473      477       +4     

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