Support '' as the argument for the setlocale emulation. Fixes #678259.

Lib/locale.py

@@ -69,7 +69,7 @@ except ImportError:
         """ setlocale(integer,string=None) -> string.
             Activates/queries locale processing.
         """
-        if value is not None and value != 'C':
+        if value not in (None, '', 'C'):
             raise Error, '_locale emulation only supports "C" locale'
         return 'C'