aggregate_spatial and raster_to_vector return AggregatePolygonResult in dry run

[JeroenVerstraelen]

This is a legacy from the very first commit of the DryRunDataCube where aggregate_spatial returns a AggregatePolygonResult instead of a DryRunDataCube. Raster_to_vector also returns an AggregatePolygonResult.

Normally we should be able to change this without breaking too many of the existing processes.

run_udf in the ProcessGraphDeserializer does contain the following check:

    if dry_run_tracer and isinstance(data, AggregatePolygonResult):
        return JSONResult({})

We should be able to just check if the current run is a dry_run instead.
Note that run_udf in the ProcessGraphDeserializer is only used for top level UDFs. It's mainly used when running UDFs on an entire vector cube.

There was also a discussion on if raster_to_vector should provide a weak_spatial_extent for the entire raster, because we don't know the geometries during the dry run.