Put back the quality of the xwiki-platform-notifications-notifiers-default module

The commit for XWIKI-16207 (https://github.com/xwiki/xwiki-platform/commit/6442e4e330776f41f8353b1db7a1652c0ea72ba4#diff-53001f6fcaca4ae26a30926433ba46cd) has reduced the quality of the module:

• By 2% for the coverage (from 19% to 17%)
• By 3% for the mutation score (from 13% to 10%)

This violates our quality rules and we need to put it back as it was.