Fix unnecessary call to organisation API endpoint

We’re caching the organisation name, but still talking to the API
to see if the organisation exists.

`Service().organisation_id` only goes to the JSON for the service.

`Service().organisation` makes a separate API call.

We only need the former to know if a service belongs to an organisation.

app/models/service.py

@@ -472,7 +472,7 @@ class Service(JSONModel):
 
     @property
     def organisation_name(self):
-        if not self.organisation:
+        if not self.organisation_id:
             return None
         return organisations_client.get_organisation_name(self.organisation_id)