firstEntry method Null safety
The entry object of the first element in the queue.
Each element of the queue has an associated DoubleLinkedQueueEntry.
Returns the entry object corresponding to the first element of the queue,
or null if the queue is empty.
The entry objects can also be accessed using lastEntry, and they can be iterated using DoubleLinkedQueueEntry.nextEntry and DoubleLinkedQueueEntry.previousEntry.
Implementation
DoubleLinkedQueueEntry<E>? firstEntry() =>
_sentinel._nextLink!._asNonSentinelEntry();