BUG: Exporting and empty LOGBOOK drawers results in missing data in the export file

[Bernt Hansen <bernt@norang.ca>]

I haven't been able to isolate this with a minimal emacs setup yet but
I'm still working on it.  I thought I should report it early anyway.

I ran into this today - if you export a file to HTML that has an empty
LOGBOOK drawer like this:

,----
| * Task to Export
|   :LOGBOOK:
|   :END:
| 
|   Lots of detail follows
`----

Then the exported file has the headline only and no detail.  Lots of
following sections are missing.  I ran into this by accidentally
clocking in a task in my documentation file, and then clocking in the
correct task within a minute - so the resulting clock duration was 0:00
and was removed by org-clock-out-remove-zero-time-clocks.

This bug seems to be dependent on a ditaa and begin_src section in the
document but pulling some of these out makes the bug go away.

My actual org-file looks like this:

,----[ org-mode.org ]
| ...
| * Time Clocking
| ** Clocking in
| *** Using the clock history to clock in old tasks
|     :LOGBOOK:
|     :END:
| 
|     You can use the clock history to restart clocks on old tasks you've clocked or to jump
|     directly to a task you have clocked previously.  I use this mainly to clock in whatever
|     got interrupted by something.
| ...
| ** Make tasks to respond to emails
| ** Create new tasks and clock in when starting something new
| * Tags, Filetags
| * Key bindings
| * GTD stuff
| ** weekly review process
| * Archiving
| ** Archiving Subtrees
| ** Archive Tag
| * Publishing
|   :PROPERTIES:
|   :CUSTOM_ID: Publishing
|   :END:
| 
| ** Playing with ditaa
| * Temporary Section to be deleted
| ...
`----

and results in the following HTML output


,----[ partial HTML output - lots of sections missing ]
| ...
| <div id="outline-container-7.2.2" class="outline-4">
| <h4 id="sec-7.2.2"><span class="section-number-4">7.2.2</span> Using the clock history to clock in old tasks </h4>
| <div class="outline-text-4" id="text-7.2.2">
| 
| 
| </div>
| </div>
| 
| </div>
| 
| <div id="outline-container-7.3" class="outline-3">
| <h3 id="sec-7.3"><span class="section-number-3">7.3</span> Playing with ditaa </h3>
| <div class="outline-text-3" id="text-7.3">
| ...
`----

If I delete the empty LOGBOOK drawer it exports properly.

-Bernt