Fix error when displaying history for a proxy whose live document has been deleted

To reproduce :

• publish a document
• remove definitively the live document
• go to the History tab of the published document and get this error

  ERROR [org.nuxeo.ecm.platform.audit.api.document.DocumentHistoryPageProvider] Error while fetching additional parameters for audit query
  org.nuxeo.ecm.core.api.ClientException: Failed to get head document for 727d2226-d749-4f47-9534-ac4f3f06963b
  	at org.nuxeo.ecm.core.api.AbstractSession.getSourceDocument(AbstractSession.java:1985)
          ...
  Caused by: org.nuxeo.ecm.core.api.DocumentException: Source document has been deleted
  	at org.nuxeo.ecm.core.api.AbstractSession.getSourceDocument(AbstractSession.java:1981)
  	... 191 more