fix(notifiers): Unable to add a notifier when no collaborator are specified

[aurelien-reeves-scalingo]

• Add a changelog entry in the section "To Be Released" of CHANGELOG.md

[aurelien-reeves-scalingo]

For info: the issue was due to the new NewStringSlice function to create a StringSlice flag. It considered the empty string as a not-nil default value. That resulted in the resulting []string being an array filed with a single empty string

[Soulou]

@aurelien-reeves-scalingo Are there other StringSlice? Otherwise LGTM

[aurelien-reeves-scalingo]

@aurelien-reeves-scalingo Are there other StringSlice? Otherwise LGTM

I did a global search to make sure all of them was fixed

[Soulou]

Great, let's merge it then!