disable sonar at ci if frontend or backend were skiped.

[mshima]

The build fails otherwise with the missing results.
Example: https://github.com/jhipster/generator-jhipster/runs/6917256799?check_suite_focus=true

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Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.

[DanielFran]

Not sure why it fails, probably still nething related with configuration...
Let's merge it!
Anyway we need to see why coverage dropped half...