Implement iteration protocol on the Missing class

This is a generalization of the fix proposed in PR #22 to the "RETURNS SETOF" without any
parameters: code may now assume that iterating over a Missing node will raise an immediate
StopIteration exception, thus basically doing nothing.

pglast/node.py

@@ -3,7 +3,7 @@
 # :Created:   mer 02 ago 2017 15:44:14 CEST
 # :Author:    Lele Gaifax <lele@metapensiero.it>
 # :License:   GNU General Public License version 3 or later
-# :Copyright: © 2017, 2018 Lele Gaifax
+# :Copyright: © 2017, 2018, 2019 Lele Gaifax
 #
 
 from enum import Enum
@@ -16,6 +16,12 @@ def __bool__(self):
     def __repr__(self):
         return "MISSING"
 
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        raise StopIteration()
+
 
 NoneType = type(None)