Default somfy_mylink shade's _attr_is_closed to None (#68053)

The base `Cover` entity requires an explicit value for
`_attr_is_closed`.

Since the `SomfyShade` is an assumed state, we don't know
by default whether the shade is open or not, so we need to
explicitly return `None` for `_attr_is_closed`

homeassistant/components/somfy_mylink/cover.py

@@ -79,6 +79,7 @@ class SomfyShade(RestoreEntity, CoverEntity):
         self._attr_unique_id = target_id
         self._attr_name = name
         self._reverse = reverse
+        self._attr_is_closed = None
         self._attr_device_class = device_class
         self._attr_device_info = DeviceInfo(
             identifiers={(DOMAIN, self._target_id)},