RFR: 8364761: (aio) AsynchronousChannelGroup.execute doesn't check null command

[Brian Burkhalter]

Add `null` check to `AsynchronousChannelGroupImpl.execute`; update existing test.

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Commit messages:
 - 8364761: (aio) AsynchronousChannelGroup.execute doesn't check null command

Changes: https://git.openjdk.org/jdk/pull/26709/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364761
  Stats: 18 lines in 2 files changed: 15 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26709.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26709/head:pull/26709

PR: https://git.openjdk.org/jdk/pull/26709