Fix crasher when revision is deleted

 * When a revision is deleted, the api doesn't return the source. This
   leaves the default null as src, which in turn gets sent to the
   tokenizer to crash. Instead, the default source should be the empty
   string.

   Ex. eswiki/Askos?oldid=68933657

Bug: 65084
Change-Id: I3853b44aada3c734ee039da3d0c063b008a983f6