Bug 20060: Resolve uninitialized warn from Koha::Template::Plugin::Branches

Bug 20060: Resolve uninitialized warn from Koha::Template::Plugin::Branches

The warn comes from undefined C4::Context->userenv->{branch}.
Can be triggered by calling mainpage.pl when being logged out in staff.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>