fix handling of null for lookup filters

[devkral]

When a lookup is detected, ignore null values. This PR is a bit conservative and also ignores UNSET but this is most probably not necessary.

This prevents django errors for null specifications and ignores null like the rest of the API does.

Is an exception for the filter exact wanted (e.g. foo_exact=null)? Shall I make a settings switch?

note: exact=null is the same like is_null=true, so there would be no functionality loss, when exact=null would not filter anymore

Types of Changes

• [ ] Core
• [x] Bugfix
• [ ] New feature
• [x] Enhancement/optimization
• [ ] Documentation

Issues Fixed or Closed by This PR

• 459

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov-commenter]

Codecov Report

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

Comparison is base (b5adb23) 87.89% compared to head (8ff2cf5) 87.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #460 +/- ## ========================================== + Coverage 87.89% 87.91% +0.01% ========================================== Files 37 37 Lines 3164 3169 +5 ========================================== + Hits 2781 2786 +5 Misses 383 383 ```

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[devkral]

fixed in an another PR