Fix: cursors get re-used when out of data

If we have reached the end of available data when paginating,
Obelisk returns a `null` cursor.
However, the library does not check for this case and keeps looping,
with said cursor set to null,
effectively duplicating the result set until the desired size is reached.

Author: Epse

src/obelisk/asynchronous/core.py

@@ -256,7 +256,7 @@ async def query(
             result_set.extend(result.items)
             params.cursor = result.cursor
 
-            if len(result_set) >= result_limit:
+            if len(result_set) >= result_limit or result.cursor is None:
                 break
 
         return result_set