Update AbstractNodeQueue.java to avoid redundant reads of head in peekNode

Avoid spinning on head when a peek/poll observes a null next but the queue is not empty.
Just for completeness’ sake: in Akka `peekNode` is also called by the enqueuer, but that is fine since that one will see all the right things (it peeks into its own store buffer); third parties may erroneously report `null` but that is not an issue for us.