refactor(sync/v3): Split load_joined_room into smaller functions

src/api/client/sync/v3/mod.rs

@@ -75,6 +75,8 @@ impl DeviceListUpdates {
 		self.changed.extend(other.changed);
 		self.left.extend(other.left);
 	}
+
+	fn is_empty(&self) -> bool { self.changed.is_empty() && self.left.is_empty() }
 }
 
 impl From<DeviceListUpdates> for DeviceLists {