Made export_to_OpenQuake_CSV export geometry of tiles w/ buildings only

export_to_OpenQuake_CSV was exporting the geometries of quadtiles irrespective of whether any buildings were being exported. However, the purpose of generating the geopackage files that contain the geometries of quadtiles when exporting to the OpenQuake CSV format is to be able to visualise results after running damage/loss calculations, and there will be no results to visualise if there is no exposure. As the GeoSummary_Tiles is already taking care of exporting the geometries of all quadtiles, including those with zero buildings, time and storage space can be saved by making this change to export_to_OpenQuake_CSV.