[jdk21u-dev] RFR: 8325730: StringBuilder.toString allocation for the empty String

[Goetz Lindenmaier]

I backport this for parity with 21.0.7-oracle.

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Commit messages:
 - Backport d2590c69b4efe5aa2b48b08070e0dbafb04ef202

Changes: https://git.openjdk.org/jdk21u-dev/pull/1387/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1387&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/jdk21u-dev/pull/1387.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1387/head:pull/1387

PR: https://git.openjdk.org/jdk21u-dev/pull/1387