Parsing does not expand COALESCE/NULLIF, results in SqlNullScalarExpression

[DanielFryeSAMC]

I've found that when attempting to parse COALESCE and NULLIF statements that the parser ends up evaluating these to a SqlNullScalarExpression with no children. As an example, for the following query:

SELECT COALESCE((SELECT TOP 1 1 FROM MYTABLE), 2)

The parser stops at COALESCE((SELECT TOP 1 1 FROM MYTABLE), 2) as a SqlNullScalarExpression with no children. The SELECT sub-query inside of that is never parsed at all.