ui: Change 'P_' to return the empty string when passed the empty string.

Reported by Mathieu Lirzin <mathieu.lirzin@etu.u-bordeaux.fr>
at <https://lists.gnu.org/archive/html/guix-devel/2015-09/msg00111.html>.

* guix/ui.scm (P_): Check whether MSGID is empty, and return it if it is.