track projection nullability through expressions

[cyberhuman]

Since we now have a basic support for NULLs I imagine it must be pretty easy to propagate information about nullability through expressions. The rules are quite simple: other than functions/operators specifically operating on nulls, everything else should be nullable if one of the operands is nullable.
NULLIF(X, Y) is always nullable
IFNULL(Xn... ) and COALESCE(Xn...) are nullable if every Xn is nullable
IS [NOT] DISTINCT FROM and <=> are not nullable
all (?) aggregate functions except for COUNT() are nullable.

[jongleb]

The current state is all works as it needs besides <=>

[ygrek]

I like the OP "pretty easy" claim 😹