Change semantics of existential subquery projection

[sileix]

Existential subquery of an expression and a projection on the same expression should have the same meaning: we should check if the value of the projection is null and undefined, if so, it should return false instead of true even if a row is returned.

It's probably fine to not check every parameter of an expression when no projection is applied - as we can assume if a query returns a row, there must be some value available in that row.