return GEOMETRYCOLLECTION EMPTY, not NULL for the query

per-file comments:
  mysql-test/r/gis.result
fix for 857050 ST_WITHIN returns wrong result with MULTIPOINT and POLYGON
        test result updated.
  sql/spatial.cc
fix for 857050 ST_WITHIN returns wrong result with MULTIPOINT and POLYGON
        return of the Geometry::envelope() changed for the empty geometry.