[lworld] Integrated: 8346702: [lworld] ShouldNotReachHere in FieldLayout::remove_null_marker()

[Frederic Parain]

Small fix to prevent calls to remove_null_marker() when processing an empty value (empty values don't have a dedicated slot for null markers, the injected field is used instead).

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Commit messages:
 - Guard remove_null_marker() against empty values

Changes: https://git.openjdk.org/valhalla/pull/1330/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1330&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346702
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1330.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1330/head:pull/1330

PR: https://git.openjdk.org/valhalla/pull/1330