Bug 23034: Remove uninitialized value warning in Mana KB settings

Bug 23034: Remove uninitialized value warning in Mana KB settings

This gets rid of the warning "Use of uninitialized value $auto_share
in string ne at [...]" by initializing the variable with empty string.

Test plan:
1) Go to page cgi-bin/koha/admin/share_content.pl and unselect "Auto
   subscription sharing". Click "Save".
2) Check plack-error.log and notice the warning "Use of uninitialized
   value $auto_share [...]"
3) Apply the patch and repeat the previous steps but now this time the
   warning is gone in plack-error.log.
4) Make sure the AutoShareWithMana syspref with and without the patch
   is in unselected state after clicking "Save".

Sponsored-by: The National Library of Finland
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 0df3fc3dae575ceb16b85de6c201f5e82d01059c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>