[WIP] Add `__isnull` Lookup Expression via Dynamic `_null` Filter Expression

[whitej6]

Closes: #1905

What's Changed

• Adds null as a filter expressions that maps to __isnull as an ORM filter

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]

Seems strange to me to be adding __isnull as a filter on text and numeric filters - generally speaking those type of fields shouldn't be null. I would have thought you'd want to add isnull support to ModelChoiceFilter and ModelMultipleChoiceFilter instead... is this specifically for handling custom fields without default values?

[whitej6]

Good catch, will look into changes needed. That was the initial source of the request.

[bryanculver]

Closed. Tracking stale PRs in #2812.