//2.4
db.test.insert({_id:0})
db.test.find({a:{$not: {$lt: null}}})
//nothing returned

//2.6
db.test.insert({_id:0})
db.test.find({a:{$not: {$lt: null}}})
{_id: 0}

Note that this works with $lt:1 or any other value, not just null