Bug 15416: Warns on guided_reports.pl

Bug 15416: Warns on guided_reports.pl

Initialising $phase variable to be '' if not defined.

To test:
1) Go to Reports -> Guided Reports Wizard
2) Notice warns
3) Apply patch
4) Refresh page
5) Confirm warns are gone and page still works as it should

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Warns are gone. Page still works as expected

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch:
-my $phase = $input->param('phase');
-$phase //= '';
+my $phase = $input->param('phase') // '';

Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com