Filter empty messages when creating claims in Swift

Messages can be removed while a claim is created, resulting in an empty
body returned from Swift. This ignores those messages.

Change-Id: I7f6500c743449abd4ce7adb6f361d31a6b8439f8

zaqar/storage/swift/utils.py

@@ -96,6 +96,8 @@ def _filter_messages(messages, filters, marker, get_object, list_objects,
             if exc.http_status == 404:
                 continue
             raise
+        if not obj:
+            continue
         obj = jsonutils.loads(obj)
         for should_skip in filters:
             if should_skip(obj, headers):