[jdk23] RFR: 8335637: Add explicit non-null return value expectations to Object.toString()

[Joe Darcy]

8335637: Add explicit non-null return value expectations to Object.toString()

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Commit messages:
 - Backport 66db71563c3ebd715a1192a9b399b618d7bdb8d0

Changes: https://git.openjdk.org/jdk/pull/20124/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20124&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335637
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20124.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20124/head:pull/20124

PR: https://git.openjdk.org/jdk/pull/20124