Fix agenda display when headlines are missing

* lisp/org-agenda.el (org-agenda-get-timestamps): Fix agenda display when headlines are missing

The following entry breaks the agenda display.

,----
| ****
|         :PROPERTIES:
|         :DateCreated: <2011-04-13 Mi 10:00>
|         :END:
|
| [[gnus:nntp%2BGwene:gwene.org.emacsen.planet][gnus:nntp+Gwene:gwene.org.emacsen.planet]]
`----

Provide the empty string as the headline if the search for the
headline returns nil.  org-format-agenda-items requires a string for
the headline parameter.

lisp/org-agenda.el

@@ -4710,7 +4710,7 @@ This function is invoked if `org-agenda-todo-ignore-deadlines',
 	    (setq hdmarker (org-agenda-new-marker)
 		  tags (org-get-tags-at))
 	    (looking-at "\\*+[ \t]+\\([^\r\n]+\\)")
-	    (setq head (match-string 1))
+	    (setq head (or (match-string 1) ""))
 	    (setq txt (org-format-agenda-item
 		       (if inactivep org-agenda-inactive-leader nil)
 		       head category tags timestr