broadcasts: initialise default values properly
broadcasts: initialise default values properly

https://florimond.dev/blog/articles/2018/08/python-mutable-defaults-are-the-source-of-all-evil/
@quis

Signed-off-by: Toby Lorne <527052641d65eef236eeef9545b2acac74c35d57@digital.cabinet-office.gov.uk>

alphagov/notifications-utils