Bug 14440: get_template_and_user can not have an empty template_name (updatesupplier.pl)

Bug 14440: get_template_and_user can not have an empty template_name (updatesupplier.pl)

Since Bug 14408, the method get_template_and_user can not have an empty template_name.
Pages calling with an empty value should use C4::Auth::checkauth()

This patch corrects acqui/updatesupplier.pl

Test plan :
- Apply patch
- Connect to intranet with a user having "vendors_manage" permission
- Go to acquisition module
- Create a new vendor
- Click on "Edit vendor"
- Change some information and save
=> Your change is saved
- Connect to intranet with a user not having "vendors_manage" permission
- Try to access <intranet>/cgi-bin/koha/acqui/updatesupplier.pl
=> Access is denied
- Disconnect from intranet
- Try to access <intranet>/cgi-bin/koha/acqui/updatesupplier.pl
=> Access is denied

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
(cherry picked from commit 015c26a5e36dae5070eab57f400237715d93ae44)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>