not() with NULL literals does not work: can't be evaluated because the expression's type is Utf8, not boolean

[alamb]

Reproducer:

> select not null;
Internal("NOT 'Literal { value: Utf8(NULL) }' can't be evaluated because the expression's type is Utf8, not boolean")

> select not(null);
Internal("NOT 'Literal { value: Utf8(NULL) }' can't be evaluated because the expression's type is Utf8, not boolean")

The correct output is null

I think the solution will likely be to teach Expr::Not about type coercion (as is done in BinaryExpr and function calls) once coercion knows about the null type (and that it can be cast to any other type). See #1184 for more details

[WinkerDu]

@alamb plz assign this issue to me, thank you.

[WinkerDu]

A raw fix here: #2144