Add 'Assumed State' property to Somfy MyLink covers (#22922)

* Explicitly return none for is_closed property

* Set the assumed_state property to true

* Added period to docstring

homeassistant/components/somfy_mylink/cover.py

@@ -59,7 +59,12 @@ class SomfyShade(CoverDevice):
     @property
     def is_closed(self):
         """Return if the cover is closed."""
-        pass
+        return None
+
+    @property
+    def assumed_state(self):
+        """Let HA know the integration is assumed state."""
+        return True
 
     @property
     def device_class(self):