Good point, but then comments are misleading (https://github.com/akka/akka/blob/master/akka-actor/src/main/java/akka/dispatch/AbstractNodeQueue.java#L56): 
next.value = null; // Null out the value so that we can GC it early

Also using the null value as a marker for an empty queue lead to failure of following test:
  "XXX" in {
    val queue = new AbstractNodeQueue[AnyRef]() {}
    queue.add(null)
    queue.peek() must_== null
    queue.count() must_== 1
    queue.isEmpty must_== false
  }