Fix the type hint for `Field.converter`

[rouge8]

Field.converter is called in Field.validateValue() on the result of Field.extractValue(), which will return either a string, JSON-serializable objects, or None, but not bytes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.07%. Comparing base (bad4cf3) to head (df757be).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk     #708   +/-   ##
=======================================
  Coverage   99.07%   99.07%           
=======================================
  Files          46       46           
  Lines        3986     3986           
  Branches      531      531           
=======================================
  Hits         3949     3949           
  Misses         23       23           
  Partials       14       14           

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