RFR: 8343801: Change string printed by nsk_null_string for null strings

[Coleen Phillimore]

Trivial change to test comment.  Ran vmTestbase/nsk/jvmti tests locally.

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Commit messages:
 - 8343801: Change string printed by nsk_null_string for null strings

Changes: https://git.openjdk.org/jdk/pull/22047/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22047&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343801
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22047.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22047/head:pull/22047

PR: https://git.openjdk.org/jdk/pull/22047