Fix crash when validating the publication of a rendition of a document

When template rendering is installed, it is possible to publish the rendition of a document (PDF for instance).
If a user that has only read access on a section publishes a pdf rendition of a document, it creates, as expected, a validation task for people having write permissions.
BUT they cannot validate as it crashes with a "Source document has been deleted" stacktrace.

Can be tested on lts.nuxeo.com and ft.nuxeo.com