Yes, but I can't think of any language where, given Optional<Foo> x and y, x.equals(y) would return null instead of either true, false, or an exception if y is

Yes, but I can't think of any language where, given Optional<Foo> x and y, x.equals(y) would return null instead of either true, false, or an exception if y is null. That's what makes SQL null-values distinct - the fact that expressions that return booleans can return NULLs.