postgis: for an empty table (with null results for extent) return early rather than allowing and printing a lexical cast failure
postgis: for an empty table (with null results for extent) return early rather than allowing and printing a lexical cast failure

mapnik/mapnik,yiqingj/work,zerebubuth/mapnik,strk/mapnik,kapouer/mapnik,Airphrame/mapnik,tomhughes/python-mapnik,mapnik/python-mapnik,strk/mapnik,mapnik/python-mapnik,CartoDB/mapnik,sebastic/python-mapnik,tomhughes/mapnik,Mappy/mapnik,Airphrame/mapnik,cjmayo/mapnik,lightmare/mapnik,stefanklug/mapnik,Airphrame/mapnik,strk/mapnik,pnorman/mapnik,jwomeara/mapnik,stefanklug/mapnik,yohanboniface/python-mapnik,strk/mapnik,naturalatlas/mapnik,mbrukman/mapnik,manz/python-mapnik,Mappy/mapnik,mapnik/mapnik,Uli1/mapnik,qianwenming/mapnik,yohanboniface/python-mapnik,davenquinn/python-mapnik,jwomeara/mapnik,sebastic/python-mapnik,tomhughes/mapnik,mapycz/python-mapnik,rouault/mapnik,pramsey/mapnik,kapouer/mapnik,cjmayo/mapnik,pnorman/mapnik,lightmare/mapnik,rouault/mapnik,stefanklug/mapnik,pramsey/mapnik,cjmayo/mapnik,pnorman/mapnik,zerebubuth/mapnik,mapnik/python-mapnik,qianwenming/mapnik,naturalatlas/mapnik,cjmayo/mapnik,tomhughes/python-mapnik,qianwenming/mapnik,mapycz/mapnik,garnertb/python-mapnik,stefanklug/mapnik,mbrukman/mapnik,pramsey/mapnik,yiqingj/work,whuaegeanse/mapnik,jwomeara/mapnik,garnertb/python-mapnik,tomhughes/mapnik,manz/python-mapnik,manz/python-mapnik,rouault/mapnik,pramsey/mapnik,Mappy/mapnik,lightmare/mapnik,Mappy/mapnik,mbrukman/mapnik,tomhughes/mapnik,qianwenming/mapnik,qianwenming/mapnik,yiqingj/work,naturalatlas/mapnik,CartoDB/mapnik,mapycz/mapnik,kapouer/mapnik,Uli1/mapnik,jwomeara/mapnik,lightmare/mapnik,zerebubuth/mapnik,mapycz/python-mapnik,kapouer/mapnik,Airphrame/mapnik,davenquinn/python-mapnik,naturalatlas/mapnik,whuaegeanse/mapnik,whuaegeanse/mapnik,CartoDB/mapnik,mapnik/mapnik,rouault/mapnik,garnertb/python-mapnik,mbrukman/mapnik,whuaegeanse/mapnik,Uli1/mapnik,mapnik/mapnik,pnorman/mapnik,yiqingj/work,davenquinn/python-mapnik,yohanboniface/python-mapnik,Uli1/mapnik,mapycz/mapnik,sebastic/python-mapnik,tomhughes/python-mapnik