Preserve mapped dates in HazardForecast.from_xarray_raster

[giovannicozzolongo]

Changes proposed in this PR:

• Preserve dates loaded from an explicit date entry in data_vars.
• Keep zero-valued forecast dates when no date mapping is supplied, with regression coverage for both paths.

This PR fixes #1305

Tests:

• python -m pytest -q climada/hazard/test/test_forecast.py (56 passed)
• python -m pytest -q climada/hazard/test/test_xarray.py climada/hazard/test/test_io.py (18 passed)
• pre-commit run --files AUTHORS.md CHANGELOG.md climada/hazard/forecast.py climada/hazard/test/test_forecast.py
• pylint climada/hazard/forecast.py climada/hazard/test/test_forecast.py (9.35/10, unchanged)

PR Author Checklist

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• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
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• Tests passing
• No new linter issues