Make _fields attr for no fields consistent with _attributes attr.
diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index 4184bc6..e068b0a 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -163,6 +163,9 @@
         self.assertEquals(x.right, 3)
         self.assertEquals(x.lineno, 0)
 
+        # this used to fail because Sub._fields was None
+        x = _ast.Sub()
+
 
 def test_main():
     test_support.run_unittest(AST_Tests)