LP#1394989: Make users_of_interest test for defined actor.usr.card values

LP#1394989: Make users_of_interest test for defined actor.usr.card values

If the open-ils.collections.users_of_interest API encounters a patron that
has a null "card" value in actor.usr the API will crash with an error like:

Received Exception:
Name: osrfMethodException
Status:  *** Call to [open-ils.collections.users_of_interest.retrieve] failed for session [1416515351.333446.1416515351406], thread trace [1]:
Can't call method "barcode" on an undefined value at /usr/local/share/perl/5.14.2/OpenILS/Application/Collections.pm line 299.

This patches users_of_interest to provide "undef" in the event that there is
not a proper barcode associated with the user when retrieving results for the
API call.

Signed-off-by: Michael Peters <mpeters@emeralddata.net>
Signed-off-by: Ben Shum <bshum@biblio.org>