Fix none nollning group test fails

[hannesblaman]

Updated one test, added one test and fixed the event signup code to pass the last test

[georgelgeback]

Do we want to allow someone to:

1. Sign up to a nollning event with a certain group
2. Edit their signup so that they don't have any group associated with the sign up

Allowing this (as we currently do, see: test_update_signup_without_group_name_is_allowed) does conflict a bit with the test_nollning_event_signup_without_group test, since you can achieve a different end state by editing the signup.