RFR: 8351344: Avoid explicit Objects.requireNonNull in String.join

[Andrey Turbanov]

We have helpful NPE messages now - they are more user-friendly.
And shorter methods are more likely to be inlined.

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Commit messages:
 - [PATCH] Avoid explicit Objects.requireNonNull where we know implicit NPE will happen
 - [PATCH] Avoid explicit Objects.requireNonNull where we know implicit NPE will happen

Changes: https://git.openjdk.org/jdk/pull/23710/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23710&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351344
  Stats: 4 lines in 1 file changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23710.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23710/head:pull/23710

PR: https://git.openjdk.org/jdk/pull/23710