8265208: [JEP-356] : SplittableRandom and SplittableGenerators - splits() methods does not throw NullPointerException when source is null

Reviewed-by: rriggs

src/java.base/share/classes/jdk/internal/util/random/RandomSupport.java

@@ -2091,6 +2091,8 @@ public class RandomSupport {
         @Override
         public Stream<SplittableGenerator> splits(long streamSize, SplittableGenerator source) {
             RandomSupport.checkStreamSize(streamSize);
+            Objects.requireNonNull(source, "source should be non-null");
+
             return StreamSupport.stream(makeSplitsSpliterator(0L, streamSize, source), false);
         }