Fix required fields for fieldOverrides

[nbadal]

This addresses some false-warnings with valid indexes JSON, see #24

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   40.78%   40.78%           
=======================================
  Files           5        5           
  Lines         152      152           
  Branches       39       39           
=======================================
  Hits           62       62           
  Misses         82       82           
  Partials        8        8           

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