[GH-523] [DDC-2074] Bugfix regarding clearing cloned PersistentCollections

This issue is created automatically through a Github pull request on behalf of jankramer:

Url: https://github.com/doctrine/doctrine2/pull/523

Message:

When calling clear on a PC that has no owner (e.g. because it was cloned), it can't be deleted as there is no metadata available (that's essentially the exception mentioned in the ticket). In these cases, I think it shouldn't be scheduled for deletion, but please have a look as my knowledge of the ORM internals is limited.