Adding a default mailer name in MessageDataCollector::getMessages() to maintain BC

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

A change was introduced in 69d6c368fc743668a2900a3ea859763719275b82 which allowed the configuration of multiple mailers.

The `MessageDataCollector::getMessages()` method now requires a `$name` parameter to fetch messages for a specific mailer, but breaks backwards compatibility by not having a default value of "default".