RFR: 8354513: Bug in j.u.l.Handler deadlock test allows null pointer during race condition

[David Beaumont]

Add null pointer guard in test formatter (since it can be called with a log record without parameters).

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Commit messages:
 - Adding null check for test formatter.

Changes: https://git.openjdk.org/jdk/pull/24619/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24619&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354513
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24619.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24619/head:pull/24619

PR: https://git.openjdk.org/jdk/pull/24619