[SQL] SAFE_CAST(NULL AS USER DEFINED type) fails SQL COMPILATION

Following SQL:

```
CREATE TYPE user_def_row AS (val ROW(i1 INT, v1 VARCHAR NULL));

CREATE MATERIALIZED VIEW v AS SELECT
SAFE_CAST(NULL AS user_def_row) AS to_roww;
```

 **error: Compiler error
Unexpected source type struct ?
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