RFR: 8324242: Avoid null check for OopHandle::ptr_raw()

[Kim Barrett]

Please review this change to use OopHandle::is_empty() rather than comparing
the result of OopHandle::ptr_raw() with nullptr.  While equivalent, the former
is the intended API for such checks.  ptr_raw should only be used directly
where it is actually needed.

Testing: mach5 tier1.

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Commit messages:
 - prefer is_empty

Changes: https://git.openjdk.org/jdk/pull/17510/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324242
  Stats: 8 lines in 3 files changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/17510.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17510/head:pull/17510

PR: https://git.openjdk.org/jdk/pull/17510