fix 2.7 failure

Author: gtema

otcextensions/sdk/sdk_resource.py

@@ -363,7 +363,7 @@ def list(cls, session, paginated=False,
             if response.status_code == 204 or \
                     (response.status_code == 200 and not response.json()):
                 # Some bad APIs (i.e. DCS.Backup.List) return emptiness
-                return None
+                return
             data = response.json()
 
             # Discard any existing pagination keys