[lworld] RFR: 8331501: [lworld] Unsafe support for null marker

[Coleen Phillimore]

Shouldn't this be getNullMarkerOffset? This was mostly implemented.  Fixed the test.

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Commit messages:
 - Fix the test.
 - Get Field's null marker offset.

Changes: https://git.openjdk.org/valhalla/pull/1317/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1317&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331501
  Stats: 8 lines in 3 files changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1317.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1317/head:pull/1317

PR: https://git.openjdk.org/valhalla/pull/1317