test(snowflake): add tests for CASE expression type annotation

[fivetran-ashashankar]

No description provided.

[georgesittas]

Discussed in Slack– let's ensure this test is included before merging:

-- Should result in an integer type
CASE WHEN score >= 90 THEN 100 ELSE NULL END;

It appears that this is currently annotated as UNKNOWN an we're looking into it.

[fivetran-ashashankar]

modified _maybe_coerce to coerce NULLs properly.
The type of a CASE expression (or any expression) that includes NULL should be the type of the non-null branches.