only run migration if we have items

pandora/item/migrations/0006_add_annotation_sequence.py

@@ -16,6 +16,7 @@ class Migration(SchemaMigration):
         ))
         db.send_create_signal('item', ['AnnotationSequence'])
         import item.models
+        if item.models.Item.objects.all().count():
             for i in item.models.Item.objects.all():
                 item.models.AnnotationSequence.reset(i)
             for a in item.models.Annotation.objects.filter(public_id=None): a.save()