templatefilters: don't stringify None into "None"

A few template keywords can in fact return None, such as {bisect}. In
some contexts, these get stringified into None instead of "". This is
leaking Python details into the UI.

mercurial/templatefilters.py

@@ -326,6 +326,8 b' def stringify(thing):'
     """
     if util.safehasattr(thing, '__iter__') and not isinstance(thing, str):
         return "".join([stringify(t) for t in thing if t is not None])
+    if thing is None:
+        return ""
     return str(thing)
 
 def strip(text):