avoid incomplete key when the unspecified guestbook_name query parameter is the empty string
avoid incomplete key when the unspecified guestbook_name query parameter is the empty string

Reviewed in https://codereview.appspot.com/9107044/

wcyuan/weather-mailer,jerarl/gcp-appengine-python,dmolson/appengine-guestbook-python,adjohnson916/appengine-guestbook-python,JackConnor/portfolio,wcyuan/weather-mailer,t9nf/appengine-guestbook-python,jerarl/gcp-appengine-python,Romzz/appengine-guestbook-python,bkeyvani/appengine-guestbook-python,artificechicago/camp2016,dmolson/appengine-guestbook-python,JackConnor/portfolio,GoogleCloudPlatform/appengine-guestbook-python,adjohnson916/appengine-guestbook-python,jeffgodwyll/appengine-flask-guestbook,t9nf/appengine-guestbook-python,JonGal/appengine-ml-demo,JonGal/appengine-ml-demo,ashwini0529/appengine-guestbook-python,bkeyvani/appengine-guestbook-python,thejeshgn/appengine-guestbook-python,ashwini0529/appengine-guestbook-python,thejeshgn/appengine-guestbook-python,jeffgodwyll/appengine-flask-guestbook,GoogleCloudPlatform/appengine-guestbook-python,Romzz/appengine-guestbook-python,artificechicago/camp2016