RFR: 8254973: CompletableFuture.ThreadPerTaskExecutor does not throw NPE in #execute

[Martin Buchholz]

8254973: CompletableFuture.ThreadPerTaskExecutor does not throw NPE in #execute

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Commit messages:
 - JDK-8254973

Changes: https://git.openjdk.java.net/jdk/pull/2018/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2018&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254973
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2018.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2018/head:pull/2018

PR: https://git.openjdk.java.net/jdk/pull/2018