Bug 10915: fix About->timeline breakage on incompletely upgraded Koha

Bug 10915: fix About->timeline breakage on incompletely upgraded Koha

This patch supplies a default value for the koha-conf.xml
setting docdir to handle the case where the user neglects
to set it upon upgrade.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Fixes dev and standard installs without docdic tag in koha-conf.xml.
Doesn't break when docdir present either. Warning still present
on packages install that didn't add the docdir definition in koha-conf.xml

Signed-off-by: Galen Charlton <gmc@esilibrary.com>