Fix explicit disabling of format assertions

[justin-tay]

Closes #1144

This also fixes the SchemaValidatorsConfig builder to initialise formatAssertionsEnabled to null instead of false.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.10%. Comparing base (48ca3c2) to head (442f456).
Report is 76 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1145      +/-   ##
============================================
- Coverage     78.90%   78.10%   -0.80%     
- Complexity     1965     2059      +94     
============================================
  Files           172      201      +29     
  Lines          6352     6851     +499     
  Branches       1255     1252       -3     
============================================
+ Hits           5012     5351     +339     
- Misses          867     1033     +166     
+ Partials        473      467       -6     

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