Fix the bug when the last code comment deleted, a review comment type comment is still in the activity list of the pull request

models/fixtures/comment.yml

@@ -102,3 +102,12 @@
   review_id: 22
   assignee_id: 5
   created_unix: 946684817
+
+-
+  id: 12
+  type: 22 # review
+  poster_id: 100
+  issue_id: 3
+  content: ""
+  review_id: 10
+  created_unix: 946684812