A ticket template is simply a badge template containing the QR code field.
You can also use this code in indico shell to create the default template:
from indico.core.db.sqlalchemy.util.management import DEFAULT_TICKET_DATA
from indico.modules.designer import TemplateType
cat = Category.get_root()
dtt = DesignerTemplate(category_id=cat.id, title='Default ticket', type=TemplateType.badge, data=DEFAULT_TICKET_DATA, is_system_template=True)
cat.default_ticket_template = dtt
db.session.commit()