Description

• Truth value handling in the PlanTyper is still buggy.

For example:
Consider:

CASE WHEN NULL THEN 1 ELSE 0 END

This returns:

ERROR : null is/are incompatible data types for the 'CASE_WHEN' operator.

According to SQL spec, the case expression should returns the value of the of the first (leftmost)
whose is true.

Per my understand, NULL should be permitted in the case expression, as NULL IS TRUE simply returns false.

See also: PostgreSQL link.

This will be a problem for us, practically because we rewrite control flow function like NULLIF to case when.

i.e.,

NULLIF(a, b)
-- is equivalent to  
CASE WHEN a=b THEN NULL ELSE a END -- a=b can return null. 

To Reproduce

CASE WHEN NULL THEN 1 ELSE 0 END
java.lang.AssertionError: ERROR:null is/are incompatible data types for the 'CASE_WHEN' operator.

Expected Behavior

Define a better handling mechanism for truth value in the typer.

Additional Context

• Java version: XXX
• PartiQL version: 0.13.2
• Add any other context about the problem here.