[5.8] MailFake::failures() returns an empty array

[shadoWalker89]

When there is no failures, calling the failures() method on the real Mailer returns an empty array.
The fake mailer should do the same.

In my application i'm using the failures() method this way.

if (in_array($some_email, $mailer->failures())) {
    // Do something
}

When running my tests and using the Mail::fake() i get this error
ErrorException : in_array() expects parameter 2 to be array, null given