Modify "changes is ready!" to "there is no change!" when appropriate

[user202729]

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit ccad5c4; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[user202729]

looks like the "deploy documentation" step is run using the workflow definition in the repository instead of in the pull request, so it doesn't show up in this pull request itself.

https://github.com/sagemath/sage/actions/runs/18261574459

you can nonetheless see the debug print at https://github.com/sagemath/sage/actions/runs/18260896785/job/51988639573?pr=40986#step:12:82

[user202729]

(Line: 28, Col: 3): The workflow must contain at least one job with no dependencies.

sure enough, it doesn't work like that. If only GitHub workflows were easier to work with.

[user202729]

should be correct now. This is surprisingly hard to get right…

the "doc-publish" workflow is triggered from the develop branch, so the effect won't be seen until it is merged into develop.

here are some test

• Test commit user202729/sage#1
• Test doc modification user202729/sage#2
  note that in the version in this pull request I don't include the debug printing or the (903 bytes) in the message.