AF-69: rename field placeholders from 'null' to 'none'

[JamieScottC]

Renames text from 'Null' to 'None for the following fields:

• Date
• Enum
• Number
• Text/String
• Dictionary fields when primary key is empty (calls primary keys toString method)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.28%. Comparing base (97a6d52) to head (b92c1e3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #141   +/-   ##
=======================================
  Coverage   99.28%   99.28%           
=======================================
  Files           9        9           
  Lines         974      974           
=======================================
  Hits          967      967           
  Misses          7        7           

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