Fix widget for boolean filters fields when generating filter form for a Dynamic Group

[jathanism]

Closes: #DNE

What's Changed

This is a quick fix based on an issue raised by @mzbroch.

• Explicitly map boolean fields to use a NullBooleanField with a widget that allows for a null selection and doesn't default to False when unchecked.

TODO

• Explanation of Change(s)
• Attached Screenshots, Payload Example
• Unit, Integration Tests
• Documentation Updates (when adding/changing features)
• Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[glennmatthews]

Fix looks reasonable. Anything we can easily do as far as unit test coverage goes?

[bryanculver]

Should target develop at this point.