RFR: 8256039: Shenandoah: runtime/stringtable/StringTableCleaningTest.java fails

[Zhengyu Gu]

If parallel cleaning does not process concurrent weak roots, it should not notify GC on dead entries (which always 0).

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Commit messages:
 - Parallel cleaning should not report dead if it does not process concurrent weak roots

Changes: https://git.openjdk.java.net/jdk/pull/1133/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1133&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256039
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1133.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1133/head:pull/1133

PR: https://git.openjdk.java.net/jdk/pull/1133