[jdk25u] RFR: 8367953: JFR sampler threads does not appear in thread dump

[Aleksey Shipilev]

Fixes another minor bug in new JFR feature added in JDK 24/25.

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `jdk_jfr`

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Commit messages:
 - Backport 64c46d8efc27911b8667c3974275c075cf79a311

Changes: https://git.openjdk.org/jdk25u/pull/291/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=291&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367953
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk25u/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/291/head:pull/291

PR: https://git.openjdk.org/jdk25u/pull/291