fix undefined/empty attribute parsing
fix undefined/empty attribute parsing

adozenlines/strophejs,sualko/strophejs,processone/strophejs,teambox/strophejs,strophe/strophejs,diegocr/strophejs,diegocr/strophejs,teambox/strophejs,metajack/strophejs,Rom4eg/strophejs,eidmanna/strophe-closure,m800-limited/strophejs,ggozad/strophejs,gabrielfalcao/strophejs,sualko/strophejs,minddistrict/strophejs,eidmanna/strophe-closure,m800-limited/strophejs,joachimlindborg/strophejs,sualko/strophejs,processone/strophejs,diegocr/strophejs,Voiceworks/strophejs,adozenlines/strophejs,damencho/strophejs,processone/strophejs,minddistrict/strophejs,ggozad/strophejs,damencho/strophejs,damencho/strophejs,minddistrict/strophejs,gabrielfalcao/strophejs,Rom4eg/strophejs,Voiceworks/strophejs,Rom4eg/strophejs,m800-limited/strophejs,ggozad/strophejs,gabrielfalcao/strophejs,joachimlindborg/strophejs,strophe/strophejs,metajack/strophejs,adozenlines/strophejs,teambox/strophejs,strophe/strophejs,Voiceworks/strophejs,joachimlindborg/strophejs