RFR: 8364761: (aio) AsynchronousChannelGroup.execute doesn't check null command [v2]

[Brian Burkhalter]

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

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8364761: Convert test to JUnit 5

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26709/files
  - new: https://git.openjdk.org/jdk/pull/26709/files/5f1ddc6b..d5d9665f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26709&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26709&range=00-01

  Stats: 59 lines in 1 file changed: 21 ins; 8 del; 30 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