Correctly compile empty do/end blocks
Correctly compile empty do/end blocks

Code like `while parse_node(t) do end` used to 
crash Compiler#process. fixes #559


bbatsov/opal,iliabylich/opal,gabrielrios/opal,jgaskins/opal,jgaskins/opal,Flikofbluelight747/opal,Flikofbluelight747/opal,kachick/opal,opal/opal,Mogztter/opal,Flikofbluelight747/opal,merongivian/opal,Mogztter/opal,wied03/opal,castwide/opal,gausie/opal,gausie/opal,jannishuebl/opal,bbatsov/opal,Ajedi32/opal,opal/opal,merongivian/opal,catprintlabs/opal,castwide/opal,iliabylich/opal,wied03/opal,suyesh/opal,c42engineering/opal,c42engineering/opal,jannishuebl/opal,kachick/opal,domgetter/opal,bbatsov/opal,catprintlabs/opal,jannishuebl/opal,gabrielrios/opal,Mogztter/opal,gabrielrios/opal,domgetter/opal,fazibear/opal,Ajedi32/opal,suyesh/opal,iliabylich/opal,merongivian/opal,suyesh/opal,fazibear/opal,kachick/opal,Ajedi32/opal,fazibear/opal,kachick/opal,gausie/opal,Mogztter/opal,c42engineering/opal,wied03/opal,catprintlabs/opal,castwide/opal,opal/opal,jgaskins/opal,opal/opal