PYTHON-2719 RawBatchCursor must raise StopIteration instead of returning empty bytes when the cursor contains no results (#624)

(cherry picked from commit 048ee81)

Author: prashantmital

pymongo/message.py

@@ -1528,7 +1528,9 @@ def raw_response(self, cursor_id=None):
                                    error_object.get("$err"),
                                    error_object.get("code"),
                                    error_object)
-        return [self.documents]
+        if self.documents:
+            return [self.documents]
+        return []
 
     def unpack_response(self, cursor_id=None,
                         codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,

test/test_cursor.py

@@ -1483,7 +1483,14 @@ def test_explain(self):
         explanation = c.find_raw_batches().explain()
         self.assertIsInstance(explanation, dict)
 
+    def test_empty(self):
+        self.db.test.drop()
+        cursor = self.db.test.find_raw_batches()
+        with self.assertRaises(StopIteration):
+            next(cursor)
+
     def test_clone(self):
+        self.db.test.insert_one({})
         cursor = self.db.test.find_raw_batches()
         # Copy of a RawBatchCursor is also a RawBatchCursor, not a Cursor.
         self.assertIsInstance(next(cursor.clone()), bytes)