For example, this query returns 3 (in QueryDslConditionsTest)

public void testCountNull() {
    QueryFactory qf = getQueryFactory();
    Query q = qf.from(getModelFactory().getUserImplClass())
            .select(Expression.count("age"))
            .build();

while only 1 value is non-null.
It might be good to behave like JPA, which ignores NULL values:
"Null values are eliminated before the aggregate function is applied, regardless of whether the keyword DISTINCT is specified." http://download.oracle.com/otn-pub/jcp/persistence-2.0-fr-eval-oth-JSpec/persistence-2_0-final-spec.pdf?AuthParam=1441633088_b920076e17d14a7f5faa4677495f61f0