Fixed behavior of ST_DumpValues(raster, ...) when passed an empty raster

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@11997 b70326c6-7e19-0410-871a-916f4a2858ee
added regression tests for ticket #2493

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@11998 b70326c6-7e19-0410-871a-916f4a2858ee