Catch potential null pointers returned by `Dataset::spatial_ref`

[weiznich]

This otherwise hits an assertion in gdal debug builds for the OSRClone method

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See https://github.com/OSGeo/gdal/blob/master/ogr/ogrspatialreference.cpp#L1406 for the relevant assertion in gdal

[lnicola]

Can you also add an assert! to SpatialRef::from_c_obj, since its argument must be a valid pointer?

[lnicola]

Thanks, will merge once GHA comes back up.