Added support for new changesets in UoW

Prevents an undefined index error if there is not already an existing changeSet for a document.

Use case:
An event subscriber for document A modifies document B and calls recomputeSingleDocumentChangeSet for document B via the UoW.

Currently if document B has not yet been modified in the current UoW when recompute is called, an undefined index error is thrown trying to access the changeSet via oid.