[Python3] return result instead of None in visitor

contributors.txt

@@ -178,3 +178,4 @@ YYYY/MM/DD, github id, Full name, email
 2017/12/01, DavidMoraisFerreira, David Morais Ferreira, david.moraisferreira@gmail.com
 2017/12/01, SebastianLng, Sebastian Lang, sebastian.lang@outlook.com
 2017/12/03, oranoran, Oran Epelbaum, oran / epelbaum me
+2017/12/27, jkmar, Jakub Marciniszyn, marciniszyn.jk@gmail.com

runtime/Python3/src/antlr4/tree/Tree.py

@@ -38,7 +38,7 @@ 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)