[jdk22u] RFR: 8325730: StringBuilder.toString allocation for the empty String

[Aleksey Shipilev]

Clean backport to fix the regression introduced in JDK 19.

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Commit messages:
 - Backport d2590c69b4efe5aa2b48b08070e0dbafb04ef202

Changes: https://git.openjdk.org/jdk22u/pull/70/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk22u&pr=70&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325730
  Stats: 25 lines in 4 files changed: 20 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk22u/pull/70.diff
  Fetch: git fetch https://git.openjdk.org/jdk22u.git pull/70/head:pull/70

PR: https://git.openjdk.org/jdk22u/pull/70