[1.8.x] Fixed incorrect session.flush() in cached_db session backend.

This is a security fix; disclosure to follow shortly.

Thanks Sam Cooke for the report and draft patch.

django/contrib/sessions/backends/cached_db.py

@@ -79,7 +79,7 @@ class SessionStore(DBStore):
         """
         self.clear()
         self.delete(self.session_key)
-        self._session_key = ''
+        self._session_key = None
 
 # At bottom to avoid circular import
 from django.contrib.sessions.models import Session  # isort:skip