Fix encoding issue in Python3.

sleekxmpp/util/misc_ops.py

@@ -19,6 +19,9 @@ def bytes(text):
     :param text: Unicode text to convert to bytes
     :rtype: bytes (Python3), str (Python2.6+)
     """
+    if text is None:
+        return b''
+
     if sys.version_info < (3, 0):
         import __builtin__
         return __builtin__.bytes(text)