Skip over bad injector class pattern

[Cambis]

Description ✍️

If you have some bad configuration like:

SilverStripe\Core\Injector\Injector:
  MyClass:
    class: null

This would throw an error as we are expecting a string here, either way we should check that the value is a a string.

DoD checklist ✅

• I have performed a self-review of my code
• My code adheres to the coding standards
• My commit messages adhere to the commit standards.
• My code has been tested.
• I have added a changeset.