AttributeNode.as_cython_attribute(): make method return value explicit

Cython/Compiler/ExprNodes.py

@@ -3557,6 +3557,7 @@ class AttributeNode(ExprNode):
         cy = self.obj.as_cython_attribute()
         if cy:
             return "%s.%s" % (cy, self.attribute)
+        return None
 
     def coerce_to(self, dst_type, env):
         #  If coercing to a generic pyobject and this is a cpdef function