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>