Return an empty string rather than None when calling repr()

Repr() should return "a string containing a printable representation of
an object," which is a convention Sphinx 1.2 relies on when building
the docs.

Fixes bug 1163369

Change-Id: Ieefd69c5b4b521500679df7745f60cb3a4d2f77c
(cherry picked from commit 29cb3251af)

horizon/utils/memoized.py

@@ -40,7 +40,7 @@ class memoized(object):
 
     def __repr__(self):
         '''Return the function's docstring.'''
-        return self.func.__doc__
+        return self.func.__doc__ or ''
 
     def __get__(self, obj, objtype):
         '''Support instance methods.'''