Fix crasher in raster when fed an empty raster (note was tempted to change the def...

Fix crasher in raster when fed an empty raster (note was tempted to change the def of empty raster to include raster with no bands but not sure where else that is used, so just revised the clip check to not try to clip if no bands).
References #3055 for PostGIS 2.4.4,
also minor doc fix references #2925 for PostGIS 2.4.4

git-svn-id: http://svn.osgeo.org/postgis/branches/2.4@16504 b70326c6-7e19-0410-871a-916f4a2858ee