missing is_closed ( rflink cover fix ) (#9776)

* Added is_closed

* whitespaces --

* removed whitespace

homeassistant/components/cover/rflink.py

@@ -103,6 +103,11 @@ class RflinkCover(RflinkCommand, CoverDevice):
         """No polling available in RFlink cover."""
         return False
 
+    @property
+    def is_closed(self):
+        """Return if the cover is closed."""
+        return None
+
     def async_close_cover(self, **kwargs):
         """Turn the device close."""
         return self._async_handle_command("close_cover")