[JEP-356] : SplittableRandom and SplittableGenerators - splits() methods does not throw NullPointerException when source is null

The split and splits methods which has SplittableRandom as source does not conform to the spec throwing NullPointerException when the source is null.

The methods behavior not conform to the spec : NullPointerException - if source is null
splits​(RandomGenerator.SplittableGenerator source)
splits(long, RandomGenerator.SplittableGenerator)