Prevent crash with empty layout box.

pdfminer/layout.py

@@ -556,10 +556,12 @@ class LTLayoutContainer(LTContainer):
             box = boxes[line]
             if box in done: continue
             done.add(box)
+            if not box.is_empty():
                 yield box
         return
 
     def group_textboxes(self, laparams, boxes):
+        assert boxes
         def dist(obj1, obj2):
             """A distance function between two TextBoxes.
             
@@ -632,6 +634,7 @@ class LTLayoutContainer(LTContainer):
             obj.analyze(laparams)
         textboxes = list(self.get_textboxes(laparams, textlines))
         assert len(textlines) == sum( len(box._objs) for box in textboxes )
+        if textboxes:
             self.groups = self.group_textboxes(laparams, textboxes)
             assigner = IndexAssigner()
             for group in self.groups: