Fix new mypy complaints related to assertGreater

Presumably due to python/typeshed#8077

tests/storage/test_monthly_active_users.py

@@ -96,8 +96,12 @@ def test_initialise_reserved_users(self):
 
         # Test each of the registered users is marked as active
         timestamp = self.get_success(self.store.user_last_seen_monthly_active(user1))
+        # Mypy notes that one shouldn't compare Optional[int] to 0 with assertGreater.
+        # Check that timestamp really is an int.
+        assert timestamp is not None
         self.assertGreater(timestamp, 0)
         timestamp = self.get_success(self.store.user_last_seen_monthly_active(user2))
+        assert timestamp is not None
         self.assertGreater(timestamp, 0)
 
         # Test that users with reserved 3pids are not removed from the MAU table
@@ -166,9 +170,11 @@ def test_user_last_seen_monthly_active(self):
         self.get_success(self.store.upsert_monthly_active_user(user_id2))
 
         result = self.get_success(self.store.user_last_seen_monthly_active(user_id1))
+        assert result is not None
         self.assertGreater(result, 0)
 
         result = self.get_success(self.store.user_last_seen_monthly_active(user_id3))
+        assert result is not None
         self.assertNotEqual(result, 0)
 
     @override_config({"max_mau_value": 5})