RFR: 8340276: Test java/lang/management/ThreadMXBean/Locks.java failed with NullPointerException

[Kevin Walls]

One more place in this test where it iterates a list of ThreadInfo and should be skipping nulls, representing threads that may have exited, or be virtual, or not exist.

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

Commit messages:
 - 8340276: Test java/lang/management/ThreadMXBean/Locks.java failed with NullPointerException

Changes: https://git.openjdk.org/jdk/pull/21056/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21056&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340276
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21056.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21056/head:pull/21056

PR: https://git.openjdk.org/jdk/pull/21056