## Description

This PR fixes form fields in order to call the `onChange` callback when the form is reset.

This change is based on the work done in https://github.com/flutter/flutter/pull/123108.

I considered adding the `onChange` callback to the `FormField` superclass but it would break existing code because two of the three subclasses defines the `onChange` callback with `ValueChanged<String>?` type and the third one defines it with `ValueChanged<String?>?`. 

## Related Issue

Fixes https://github.com/flutter/flutter/issues/123009.

## Tests

Adds 3 tests.