DocumentModel.refresh() shouldn't fail on not-yet-created doc

When doing doc.refresh() on a DocumentModel for which session.createDocument has not been called yet, we shouldn't fail.
The semantics are "update to data saved by other docs" but there are none so it's ok to do nothing.

org.nuxeo.ecm.core.api.ClientException: Failed to get refresh data
at org.nuxeo.ecm.core.api.AbstractSession.refreshDocument(AbstractSession.java:3281)
at org.nuxeo.ecm.core.api.impl.DocumentModelImpl.refresh(DocumentModelImpl.java:1582)
...
Caused by: org.nuxeo.ecm.core.api.WrappedException: Exception: org.nuxeo.ecm.core.model.NoSuchDocumentException. message: No such document: No such document: /doc
at org.nuxeo.ecm.core.storage.sql.coremodel.SQLSession.resolvePath(SQLSession.java:241)
at org.nuxeo.ecm.core.api.DocumentResolver.resolveReference(DocumentResolver.java:68)
at org.nuxeo.ecm.core.api.AbstractSession.resolveReference(AbstractSession.java:523)
at org.nuxeo.ecm.core.api.AbstractSession.refreshDocument(AbstractSession.java:3254)
... 22 more