Blank note content in HTML format causes server oops during HTML to TomboyXML conversion

STR:

1. Create a note in Tomboy which has title but no content
2. Sync the note to couchdb
3. Modify note_format to be non-xml (i.e. emulate older behavior with HTML in content node)
4. Try to sync the notes:

ERROR:root:WSGI Handled Exception
Traceback (most recent call last):
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/django/core/handlers/base.py", line 86, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/ubuntuone/web/oauth/decorators.py", line 28, in wrapper
    return func(request, *args, **kwargs)
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/ubuntuone/account/djangolib/decorators.py", line 30, in wrapper
    return func(request, *args, **kwargs)
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/ubuntuone/web/notes/views.py", line 250, in api_user_info
    (_, last_sync_revision) = get_notes_for_tomboy (request, None, True, -2)
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/ubuntuone/web/notes/views.py", line 306, in get_notes_for_tomboy
    note_to_add["note-content"] = HTMLToTomboy(note["content"])
  File "/home/rtg/canonical/ubuntuone/trunk-2a/utilities/../lib/ubuntuone/web/notes/tomboyxml.py", line 159, in HTMLToTomboy
    html_tree = etree.parse(StringIO.StringIO(html), etree.HTMLParser())
  File "lxml.etree.pyx", line 2583, in lxml.etree.parse (src/lxml/lxml.etree.c:25057)
  File "parser.pxi", line 1482, in lxml.etree._parseDocument (src/lxml/lxml.etree.c:63677)
  File "parser.pxi", line 1510, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:63925)
  File "parser.pxi", line 1389, in lxml.etree._parseDoc (src/lxml/lxml.etree.c:62857)
  File "parser.pxi", line 931, in lxml.etree._BaseParser._parseDoc (src/lxml/lxml.etree.c:60016)
  File "parser.pxi", line 542, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:56659)
  File "parser.pxi", line 628, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:57504)
  File "parser.pxi", line 579, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:57049)
XMLSyntaxError: None