Bug 19937: Silence warnings t/db_dependent/www/batch.t

Bug 19937: Silence warnings t/db_dependent/www/batch.t

TEST PLAN
---------
1) Run the following on a kohadevbox:
    git checkout -b bug_19937 origin/master
    sudo koha-shell -c bash kohadev
    prove t/db_dependent/www/batch.t
    cat /var/log/koha/kohadev/plack-error.log

    The following errors are triggered at the end of the log file:
        Use of uninitialized value in array element at
        /home/vagrant/kohaclone/tools/batch_records_ajax.pl line 50.
        Use of uninitialized value $results_per_page in numeric eq (==) at
        /home/vagrant/kohaclone/tools/batch_records_ajax.pl line 53.
        Use of uninitialized value in uc at
        /home/vagrant/kohaclone/C4/ImportBatch.pm line 1120.

2) Run the following on a kohadevbox:
    exit
    git bz apply 19937
    restart_all
    sudo koha-shell -c bash kohadev
    prove t/db_dependent/www/batch.t
    cat /var/log/koha/kohadev/plack-error.log

    The log file will end with the restarting of plack, not the
    errors.

3) run koha qa test tools

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended handling of $results_per_page.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>