The value NULL is a very tricky subject in the database world, so don't be surprised if several applicants trip up on this question.
The value NULL means UNKNOWN; it does not mean '' (empty string). Assuming ANSI_NULLS are on in your SQL Server database, which they are by default, any comparison to the value NULL will yield the value NULL. You cannot compare any value with an UNKNOWN value and logically expect to get an answer. You must use the IS NULL operator instead.