Bug 25506: Fix for "Use of uninitialized value" in about.pl

Bug 25506: Fix for "Use of uninitialized value" in about.pl

Perl warning on the "About Koha" page:

Use of uninitialized value in string eq
at /home/vagrant/kohaclone/about.pl line 220.

Fixed by adding additional precheck for
C4::Context->config('zebra_auth_index_mode') being Perl's "true".

To test:
    1) Ensure you don't have "<zebra_auth_index_mode>...
       </zebra_auth_index_mode>" in your koha-conf.xml so you will have
       "C4::Context->config" returning "undef"
    2) Go to the "About Koha" page.
    3) Observe the warning in the log file.
    4) Apply patch.
    5) Repeat step 2.
    6) Check that previous warning was suppressed.

Signed-off-by: Andreas Roussos <a.roussos@dataly.gr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 67ff0f72854d4dabdda8cb59696f15e7b5d88c2a)
Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
(cherry picked from commit 95c56339b9d0de7a8b5d10a77b52917ec79ff1fe)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>