Feat: add isnull not_isnull for m2m filter

[lozzo]

Add isnull not_isnull for m2m field filter

Description

Original m2m_filter dont support is null operator and = null is not eq to is null

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[abondar]

Hello!

Please add tests demonstrating new working behaviour, which would fail on old version