Fixed #25882 -- Prevented fast deletes matching no rows from crashing on MySQL.

Thanks to Trac aliases gerricom for the report, raphaelmerx for the
attempts to reproduce and Sergey Fedoseev and Tim for the review.

Refs #16891

django/db/models/sql/subqueries.py

@@ -68,7 +68,7 @@ class DeleteQuery(Query):
                 # We can't do the delete using subquery.
                 values = list(query.values_list('pk', flat=True))
                 if not values:
-                    return
+                    return 0
                 return self.delete_batch(values, using)
             else:
                 innerq.clear_select_clause()