Bug 27745: prevent warning for uninitialized value as a hash key

Bug 27745: prevent warning for uninitialized value as a hash key

When you edit and save items with an empty "c - Shelving location"
field, it gets stored into the database as "NULL",
which is allowed in the database, and the field is not "required",
so this seems expected.

But when the search page is rendered, it gets value by location key
($shelflocations->{ $item->{location} }) which gives undef warning.

Solved by checking if $item->{location} is perl true.

To reproduce:
    1) Edit the book or create new one with an empty shelving location.
    2) Use the search feature to have that book in the searched list.
    3) Check plack-intranet-error.log to find "Use of uninitialized
value in hash element" error.
    4) Apply the patch.
    5) Repeat the search and check the logs again to ensure that error
didn't appear again.

Signed-off-by: Didier Gautheron <didier.gautheron@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>