Fixed small bug with question mark operator, wasn't adding the empty state's output

main.go

@@ -158,7 +158,7 @@ func thompson(re string) *State {
 			s2 := &State{}
 			s2.transitions = make(map[int][]*State)
 			s2.content = EPSILON
-			s2.output = make([]*State, 0)
+			s2.output = append(s2.output, s2)
 			s2.isEmpty = true
 			s3 := alternate(s1, s2)
 			nfa = append(nfa, s3)