[1.7.x] Fixed #22710 -- Documented aggregation behavior for empty QuerySets.

Thanks matklad for the report.

Backport of 7c6e96ec1f from master

docs/ref/models/querysets.txt

@@ -2659,6 +2659,12 @@ aggregate functions, see
     aggregation on date/time fields in SQLite will raise
     ``NotImplementedError``.
 
+.. admonition:: Note
+
+    Aggregation functions will return ``None`` when used with an empty
+    ``QuerySet``. For example, the ``Count`` aggregation function will return
+    ``None`` instead of ``0`` if the ``QuerySet`` contains no entries.
+
 Avg
 ~~~