Chore(optimizer): add annotation test for IFNULL

[fivetran-felixhuang]

IFNULL is already implemented. Adding test

https://docs.snowflake.com/en/sql-reference/functions/ifnull

Platform	Supported	Argument Type	Return Type
Snowflake	✅ Yes	IFNULL(expr1, expr2) - two expressions of compatible type	Same type as the returned expression (expr1 if not NULL, otherwise expr2)
BigQuery	✅ Yes	IFNULL(expression, value_if_null) - two compatible type expressions	Type of first non-NULL argument
Redshift	❌ No	N/A - Use COALESCE(expr1, expr2) or NVL(expr1, expr2) instead	N/A
PostgreSQL	❌ No	N/A - Use COALESCE(expr1, expr2) instead	N/A
Databricks	✅ Yes	IFNULL(expr1, expr2) - returns expr2 if expr1 is NULL, otherwise expr1	Type of returned expression
DuckDB	✅ Yes	ifnull(expr, other) - two arguments, returns 'other' if 'expr' is NULL	Type of returned argument
TSQL	❌ No	N/A - Use ISNULL(expr, replacement) or COALESCE(expr1, expr2) instead	N/A