Synology sensor quick return if attr is null (#21709)

* Quick return if attr is null

There are some case where attr is null. Returning null doesn't change anything (in my case this is mapped to a volume that doesn't exist, not sure what others are seeing). 

If you have confirmed you hass instance for C instead of F you do not see this error.

* update == to is

* whitespace

homeassistant/components/sensor/synologydsm.py

@@ -230,6 +230,9 @@ def state(self):
                 attr = getattr(
                     self._api.storage, self.var_id)(self.monitor_device)
 
+                if attr is None:
+                    return None
+
                 if self._api.temp_unit == TEMP_CELSIUS:
                     return attr