fix "mark list" with an empty list

mark.go

@@ -40,6 +40,9 @@ func MarkList(state *BrowserState) error {
 		}
 	}
 	state.Modal = buf.Bytes()
+	if len(state.Modal) == 0 {
+		state.Modal = []byte("(empty)\n")
+	}
 	return Print(state)
 }