RFR: 8335637: Add explicit non-null return value expectations to Object.toString() [v4]

[Joe Darcy]

> Make well-behaved implementation expectations of Object.{toString, hashCode} explicit.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Narrow scope of the change.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20063/files
  - new: https://git.openjdk.org/jdk/pull/20063/files/66679fb8..ee007c3e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20063&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20063&range=02-03

  Stats: 13 lines in 1 file changed: 0 ins; 13 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20063.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20063/head:pull/20063

PR: https://git.openjdk.org/jdk/pull/20063