Bug 15270: Make Koha::Objects->find return undef instead of crashing

Bug 15270: Make Koha::Objects->find return undef instead of crashing

On 3.20.05, if AnonymousPatron is set to a nonexistent borrowernumber, the about.pl crashes with

  DBIC result type  isn't of the type Borrower at /home/koha/src/Koha/Objects.pm line 87.

This was fixed on master by
  commit 6882949b1b3bd1284e3d2877244a64edee3883ca
  Date:   Wed Apr 8 06:38:34 2015 -0400
      Bug 13967 - System preferences need a package

Test plan:
Set the AnonymousPatron pref to a nonexistent borrowernumber
Go on about.pl
Without this patch, you get the error.

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Set AnonymousPatron pref to a nonexistent borrowernumber. Before the
patch, about.pl shows a nasty error. After applying the patch the
error is gone. Tests in t/db_dependent/Objects.t pass.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>