Fix panic when database is not specified
Fix panic when database is not specified

This commit also makes the public database as the default database.

rickmak/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,rickmak/skygear-server