Bug 20616: Prevent "edit serials" form submission if no serial is selected

Bug 20616: Prevent "edit serials" form submission if no serial is selected

In order to prevent the form to submit and gives an ugly error if no
issues are selected we will make sure at least one is checked otherwise
an alert will be displayed.

Test plan:
- create a subscrpition
- go to the serials collection page
- don't check any checkboxes for serials
- click 'Edit serials'

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>