Bug 16009: fix GetMember() search on NULL/undef values

Bug 16009: fix GetMember() search on NULL/undef values

This patch fixes a bug whereby GetMember(searchfield => undef)
(i.e., searching for patron records where 'searchfield' is NULL)
would crash.

This fixes a regression introduced by bug 15344 that in turn
exposed a long-standing bug in GetMember().

To test:

[1] Import some offline circulation transactions that include at
    least one return.
[2] Attempt to view the list of pending transactions; a crash
    will occur.
[3] Apply the patch and view the list of pending transactions again;
    this time, it should work.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>