[1.9.x] Fixed #22705 -- Fixed QuerySet.bulk_create() on models without any fields on Oracle.

Fixed on other backends by 134ca4d438.

Thanks Mariusz Felisiak for the solution.

Backport of 7a5b7e35bf from master

django/db/backends/oracle/operations.py

@@ -267,6 +267,9 @@ WHEN (new.%(col_name)s IS NULL)
     def max_name_length(self):
         return 30
 
+    def pk_default_value(self):
+        return "NULL"
+
     def prep_for_iexact_query(self, x):
         return x