[Python2] return result instead of None in visitor

runtime/Python2/src/antlr4/tree/Tree.py

@@ -40,7 +40,7 @@ class ParseTreeVisitor(object):
         n = node.getChildCount()
         for i in range(n):
             if not self.shouldVisitNextChild(node, result):
-                return
+                return result
 
             c = node.getChild(i)
             childResult = c.accept(self)