RFR: 8335591: Fix -Wzero-as-null-pointer-constant warnings in ConcurrentHashTable

[Kim Barrett]

Please review this trivial change to ConcurrentHashTable.  Initialization of
and assignments to the _invisible_epoch member are changed from a value of 0
to nullptr.  This removes some -Wzero-as-null-pointer-constant warnings when
building with that enabled.

Testing: mach5 tier1

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Commit messages:
 - fix _invisible_epoch usage in ConcurrentHashTable

Changes: https://git.openjdk.org/jdk/pull/19996/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335591
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19996.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19996/head:pull/19996

PR: https://git.openjdk.org/jdk/pull/19996