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.