Bug 28561: Fix noisy warning about $direction too

Bug 28561: Fix noisy warning about $direction too

Use of uninitialized value $direction in string ne at /usr/share/koha/opac/opac-shelves.pl line 265.

Bonus:
Use of uninitialized value $sortfield in string eq at /usr/share/koha/opac/opac-shelves.pl line 264.

Test plan:
While testing patch 1, check the logs for these warnings with and
without this patch.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>