Ah right, my .exists suggestion wasn't thinking of the embedding language.
In the embedded query, you would need to follow the same rules as SQL, as far as I know. Which means that if you are doing an equality test, as in your example, NULL will behave as false, and no extra test is needed. For clarity you could decide to include it, though.
But if you are doing an inequality test, you would need the equivalent of
x IS NULL OR x <> literal
i.e.
x.isEmpty || x =!= literal
Because, as in SQL, a NULL would result in the inequality appearing false, when it should make the inequality true.
--Toby