fix: filter nulls in polars df to align with subsequent check of coeffs

[FabianHofmann]

@lkstrp would that resolve the issues to encountered with pypsa multi horizon. you can also point me out the example and I can check

Changes proposed in this Pull Request

• also filter null coeffs in polars dataframe filtering

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[lkstrp]

Na, they don't. Just install this branch in PyPSA (so unpin linopy 0.5.7) and run test/test_lopf_multiinvest.py