Added an integration test for #4427 (#4428)

* Added an integration test for #4427.

* Skip if not redis.

* fixup! Skip if not redis.

t/integration/test_canvas.py

@@ -111,6 +111,18 @@ class test_chain:
 
 class test_group:
 
+    @flaky
+    def test_empty_group_result(self, manager):
+        if not manager.app.conf.result_backend.startswith('redis'):
+            raise pytest.skip('Requires redis result backend.')
+
+        task = group([])
+        result = task.apply_async()
+
+        GroupResult.save(result)
+        task = GroupResult.restore(result.id)
+        assert task.results == []
+
     @flaky
     def test_parent_ids(self, manager):
         assert manager.inspect().ping()