Bug 11175: (follow-up) Don't return explicitly undef

Bug 11175: (follow-up) Don't return explicitly undef

This would create in list context an list of one element that is
undef, which we don't want, we want empty list if there are no
components.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Pasi Kallinen <pasi.kallinen@koha-suomi.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Andrew Nugged <nugged@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>