[jdk19] RFR: 8289570: SegmentAllocator:allocateUtf8String(String str) default behavior mismatch to spec

[Maurizio Cimadamore]

This simple patch fixes an issue where the API implementation for `SegmentAllocator::allocateUtf8String` does not conform to the javadoc implSpec.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk19/pull/99/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=99&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289570
  Stats: 20 lines in 2 files changed: 19 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk19/pull/99.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/99/head:pull/99

PR: https://git.openjdk.org/jdk19/pull/99