fix: fields with json type display 'null'

[joebuono]

Problem

In CMS, if the customer creates an empty record and then reopens the record, 'null' is displayed for JSON fields.

Solution

Adds a condition to avoid stringifying null. This change also applies to metadata and nonModelField types as well as json.

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[codecov-commenter]

Codecov Report

Merging #1051 (b702d38) into main (694d214) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main    #1051   +/-   ##
=======================================
  Coverage   93.86%   93.86%           
=======================================
  Files         124      124           
  Lines        5790     5790           
  Branches     1747     1747           
=======================================
  Hits         5435     5435           
  Misses        337      337           
  Partials       18       18           

Files Changed	Coverage Δ
...react/lib/forms/form-renderer-helper/form-state.ts	98.93% <ø> (ø)

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