Avoid exception in PubMed plugin when response is empty.
Avoid exception in PubMed plugin when response is empty.

jondo/paperpile,jondo/paperpile,pombredanne/paperpile,jondo/paperpile,jondo/paperpile,jondo/paperpile,pombredanne/paperpile,pombredanne/paperpile,pombredanne/paperpile,pombredanne/paperpile