Amplify Github App posts empty preview link on the PR even if deployment was skipped because of `AMPLIFY_DIFF_DEPLOY`

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Deployments, Frontend builds, Git providers, Web previews

### Is your feature request related to a problem? Please describe:

We are using Amplify Hosting with a connected GitHub provider.

When using Amplify Hosting with GitHub Pull Request previews, and when `AMPLIFY_DIFF_DEPLOY` is enabled, deployments are skipped as expected if no changes are detected. However, the GitHub Amplify App still posts a preview URL in Pull Request comments. This URL leads to a 404 error (or the Amplify welcome page) because nothing was deployed.

### Describe how you'd like this feature to work

Amplify PR Preview URL should be posted only if deployment happened.

Start by tracing the GitHub Amplify App flow that posts pull request preview URLs and the handling of AMPLIFY_DIFF_DEPLOY when deployment is skipped. No files or tests are named in the issue; done means no preview URL is posted when no deployment occurs, while URLs continue to be posted after deployments.

Written by the indexing model from the issue text.