Allow to clean orphaned media when the registerMediaConversionsUsingModelInstance property is set to true on the model

[tomcoonen]

This is a fix for the media-library:clean command.

Without the check if the model exists, the $model->mediaConversions = [] would fail because $model would be null.

When $model->registerMediaConversionsUsingModelInstance is false, the $media->model check will not execute and not result in an extra query.

[freekmurze]

Thank you!