Bug 36098: Default to 'file' if pref does not exist

Bug 36098: Default to 'file' if pref does not exist

During the installer process there is a bunch of warnings
  "Use of uninitialized value $storage_method in string eq at"

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit e2440f2c617c48d77cc416857937b6fb18c34208)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit a42bce58b21c32868057dc0a45dde7580efae8f9)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 922928eb7def8016abdcb4a8b8396858421b9f71)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>