fix: fill missing irradiation with zero for trigon_model="other"

[fneum]

Changes proposed in this Pull Request

With trigon_model="other", TiltedIrradiation returned NaN in cells without irradiation data, e.g. north of 65° N in SARAH cutouts. Any aggregated time series (layout, matrix, shapes) that includes such a cell became NaN for all time steps.

The "simple" branch already fills missing direct, diffuse and ground irradiation with zero. This PR does the same for the other branch (one line), so both models treat missing data the same way.

Note: zero-filling means cells without data contribute no generation rather than marking the result as unknown. This is the existing behaviour of the default "simple" model.

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).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (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.