[FIX] tools.ustr, like str, must always return a string

lp bug: https://launchpad.net/bugs/311595 fixed

bzr revid: christophe@taupe-20081226184538-lkf5lepssmgctbsz

bin/tools/misc.py

@@ -649,10 +649,6 @@ def ustr(value):
     @return: unicode string
     """
 
-    if (value is None) or (value is False):
-        return value
-    if not value:
-        return u''
     if isinstance(value, unicode):
         return value