Particularly for contains(null), and containsKey/containsValue for Maps.

The general rule seems to be, for collections that disallow nulls:

Queues and Deques: contains(null) returns false.

Sets: contains(null) throws NPE.

Maps: containsKey(null) and containsValue(null) throw NPE.

For unmodifiable collections tests, add:

 - List.containsAll(..., null, ...)
 - Set.containsAll(..., null, ...)
 - Map.getOrDefault(null, ...)
 - entrySet, keySet, and values views contains(null) and containsAll(..., null, ...)