Fix #3253: In Py2 environment, building another locale with a non-captioned toctree produces None captions

CHANGES

@@ -11,6 +11,8 @@ Bugs fixed
 ----------
 
 * #3246: xapian search adapter crashes
+* #3253: In Py2 environment, building another locale with a non-captioned
+  toctree produces `None` captions
 
 Release 1.5.1 (released Dec 13, 2016)
 =====================================

sphinx/addnodes.py

@@ -49,7 +49,7 @@ class toctree(nodes.General, nodes.Element, translatable):
     """Node for inserting a "TOC tree"."""
 
     def preserve_original_messages(self):
-        if 'caption' in self:
+        if self.get('caption'):
             self['rawcaption'] = self['caption']
 
     def apply_translated_message(self, original_message, translated_message):