Returns "there are no ballots to view yet" rather than 404ing when the ballot link is followed for a document that exists.

It is typical for last call to be issued (which includes a link to the ballot page) well in advance of creating any ballot for a document.
Fixes bug 1233
Commit ready for merge
 - Legacy-Id: 7077

Author: rjsparks

ietf/doc/views_doc.py

@@ -650,7 +650,7 @@ def document_ballot_content(request, doc, ballot_id, editable=True):
         ballot = all_ballots[-1]
 
     if not ballot:
-        raise Http404
+        return "<p>No ballots are available for this document at this time.</p>"
 
     deferred = doc.active_defer_event()