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

[Jim Laskey]

Add check for null.

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Commit messages:
 - Check for null source
 - Make makeXXXSpliterator final
 - Move makeXXXSpilterator from public (@hidden) to protected. No API change.

Changes: https://git.openjdk.java.net/jdk/pull/3495/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3495&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265208
  Stats: 178 lines in 4 files changed: 28 ins; 131 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3495.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3495/head:pull/3495

PR: https://git.openjdk.java.net/jdk/pull/3495