adjust test_reset_email_user_deleted for None instead of str

test/unit/app/managers/test_UserManager.py

@@ -240,7 +240,7 @@ def test_reset_email_user_deleted(self):
         self.user_manager.delete(user)
         assert user.deleted is True
         message = self.user_manager.send_reset_email(self.trans, {"email": user_email})
-        assert message == "Failed to produce password reset token. User not found."
+        assert message == None
 
     def test_get_user_by_identity(self):
         # return None if username/email not found