Reported by Gottfried at
<https://lists.gnu.org/archive/html/help-guix/2023-04/msg00000.html>.
* guix/build/po.scm (xref-regexp): Dispatch on the number of
arguments. Ensure there is no info-manual argument or it is empty.
(translate-cross-references): Adapt to changed regexp.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>