CamelIMAPXNamespace: Allow NIL separator character.

RFC 2342 allows it:

  Namespace_Response = "*" SP "NAMESPACE" SP Namespace SP Namespace
                       SP Namespace

  Namespace = nil / "(" 1*( "(" string SP  (<"> QUOTED_CHAR <"> /
              nil) *(Namespace_Response_Extension) ")" ) ")"
              ^^^