only prepend an image path to CreateBundle if MediaBundle is not present

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

as per discussion in https://github.com/symfony-cmf/MediaBundle/issues/36#issuecomment-23643093