RFR: 8358526: Clarify behavior of java.awt.HeadlessException constructed with no-args

[Phil Race]

Clarify the behaviour of new HeadlessException().getMessage()
The spec. is updated to be clear that empty means null, not an empty string.

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Commit messages:
 - 8358526

Changes: https://git.openjdk.org/jdk/pull/25881/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25881&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358526
  Stats: 52 lines in 2 files changed: 51 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25881.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25881/head:pull/25881

PR: https://git.openjdk.org/jdk/pull/25881