[SQL] CAST(VARIANT AS VARCHAR) returns NULL for a BOOLEAN variant

Given the following SQL:

```
CREATE TABLE varnt_tbl(
id INT,
booll BOOL);

CREATE MATERIALIZED VIEW v1 AS SELECT
id,
CAST(booll AS VARIANT) AS booll
FROM varnt_tbl;

CREATE MATERIALIZED VIEW v2 AS SELECT
id,
CAST(booll AS VARCHAR) AS  booll
FROM v1;
```

View v2 produces the following result:
**[{'id': 0, 'booll': None}]**

Example input: `{"id": 0, "booll": False}`

