fix(core): accidentally adding an extra comma silently disables all modules

[IvanEh]

I have recently been working on vuestorefront and found an issue with their public repository https://github.com/vuestorefront/template-commercetools (pull request created). There was an extra comma that made the application to infinitely loop because the server middleware wasn't handling the request due to a missing nuxt module.

The small typo led to an embarrassingly frustrating result that was not easy to solve.

With my change, I'm skipping the handler validation if there was no handler which seems very sensible to me

Types of changes

• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly. (PR: #)
• [x] I have added tests to cover my changes (if not applicable, please state why)
• [x] All new and existing tests are passing.