fix(web): raster and raster-dem source

[josxha]

RasterSource and RasterDemSource send null for tiles or url if not set since recently. This throws an error in maplibre gl js. This pr only sets those parameters if not null.

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Pull request overview

This PR fixes an issue where RasterSource and RasterDemSource were sending null values for tiles or url parameters to MapLibre GL JS, which was throwing errors. The solution splits each factory constructor into two separate private constructors - one for URL-based sources and one for tiles-based sources, conditionally selecting between them based on which parameter is non-null.

Changes:

• Refactored SourceSpecification.rasterDem from an external factory to a regular factory that delegates to either _rasterDemUrl or _rasterDemTiles based on whether url is non-null
• Refactored SourceSpecification.raster from an external factory to a regular factory that delegates to either _rasterUrl or _rasterTiles based on whether url is non-null

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.33%. Comparing base (1a0b042) to head (f6fd0c8).
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