Chore(optimizer): add annotation tests for NVL

[fivetran-felixhuang]

NVL is already implemented. Adding some annotation tests

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

Platform	Supported	Argument Type	Return Type
Snowflake	Yes	Any expressions (ideally same type)	Type of returned expression
BigQuery	No	N/A	N/A
Redshift	Yes	Any expressions	Determined by compatible input types
PostgreSQL	No	N/A	N/A
Databricks	Yes	expr1, expr2 (any type)	Same type as arguments
DuckDB	No	N/A	N/A
TSQL	No	N/A	N/A

[VaggelisD]

I think I'd keep only 2-3 of these tests and maybe try out what happens if we supply mixed types such as NVL(1, 3.5, ...) etc. Does that work in Snowflake? Would we still annotate it properly?

[VaggelisD]

This is tested already from the first test

Suggested change

	# dialect: snowflake
	NVL('hello', 'fallback');
	VARCHAR;

[fivetran-felixhuang]

thanks for pointing this out, let me remove it