(v2.2.x) Fix empty destination path calculation for validation

[abelsromero]

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Documentation
• [ ] Refactor
• [ ] Build improvement
• [ ] Other (please describe)

What is the goal of this pull request?

Fix #728 Also improve message. We cannot get the exact name without duplication asciidoctor core logic, but now when outputFile is not set simply wildcard will be displayed instead of the source extension to avoid confusion:

[warn] Duplicated destination found: overwriting file: /.../path/single-output.*

Are there any alternative ways to implement this?

We could only check for toDir (as done in main branch) but that would break builds using older AsciidoctorJ versions.

Are there any implications of this pull request? Anything a user must know?

no

Is it related to an existing issue?

- [x] Yes - [ ] No *Finally, please add a corresponding entry to CHANGELOG.adoc*