Fix missing file error in Guidebook export

Author: kitsuta

uber/models/guests.py

@@ -223,7 +223,7 @@ def guidebook_data(self):
 
     @property
     def guidebook_images(self):
-        if not self.bio:
+        if not self.bio or not self.bio.pic_filename:
             return ['', ''], ['', '']
 
         prepend = sluggify(self.group.name) + '_'