Casting explicit NULL for way_pixels/way_area columns to numeric type

As discussed in #5227: For avoiding errors and unexpected side effects we should cast explicit `NULL` values of `way_pixels`/`way_area` columns to a numeric type. This occurs mostly in queries combining point and polygon features using `UNION ALL`. Like: https://github.com/openstreetmap-carto/openstreetmap-carto/blob/4c076333b773a022a219bb60730a428902dd4f2c/project.mml#L1392-L1439