NULL typing in BigQuery CTEs - google-bigquery

Question

I want to execute a merge statement in BigQuery which fills missing fields coming from late data of an upstream data pipeline. For this, I dynamically create the SQL merge statement using a CTE. Since it is late data, I know that the target table already has a partially filled row. So when creating the dynamic SQL merge statement I utilize this information to only update the missing fields by setting the already present fields in the CTE as NULL, using T.already_filled_col= IFNULL(S.already_filled_col, T.already_filled_col), where S.already_filled_col is NULL.
However, since it is a CTE without a schema, BigQuery complains that the NULL value in the CTE is of the wrong type. In particular, I get the error
No matching signature for function IFNULL for argument types: INT64, STRING
How do I specify the type of the NULL value in a CTE?
Below is a minimal working example
Given a target table with the following schema
[
{
"name": "join_col",
"type": "STRING",
"mode": "NULLABLE"
},
{
"name": "already_filled_col",
"type": "STRING",
"mode": "NULLABLE"
},
{
"name": "update_col",
"type": "STRING",
"mode": "NULLABLE"
}
]
Fill it with dummy data:
INSERT `my_project.my_dataset.test_table` (join_col, already_filled_col, update_col)
VALUES
('join_key', 'on_time_data', NULL)
My merge statement:
MERGE `my_project.my_dataset.test_table` AS T
USING (
SELECT
"join_key" AS join_col,
NULL AS already_filled_col,
"late_data" AS update_col
) AS S
ON T.join_col= S.join_col
WHEN MATCHED THEN
UPDATE SET
T.already_filled_col= IFNULL(S.already_filled_col, T.already_filled_col),
T.update_col= IFNULL(S.update_col, T.update_col)
I am aware that I could replace NULL AS already_filled_col in the CTE with CAST(NULL AS STRING) AS already_filled_col. However, I am hoping that there is an easier way, since for the actual data the types are not always STRING and deriving them dynamically is something I was hoping to avoid.