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

[Harshitha Onkar]

On Wed, 18 Jun 2025 17:57:26 GMT, Phil Race <prr at openjdk.org> wrote:

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

Updated javadoc and regression test LGTM.

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Marked as reviewed by honkar (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25881#pullrequestreview-2947358998