[5.8] Only remove the event mutex if it was created

[lorisleiva]

The purpose of this PR is to fix an issue that throws PHP Fatal errors when running the scheduler in a test.

Description of the issue

• Given a scheduled CallbackEvent with a description (i.e. using the name() method)
• When we run the scheduler in a test using $this->artisan('schedule:run')
• Then the test passes but it throws PHP Fatal errors (see image below)

Note that, since $schedule->job() is syntactic sugar for $schedule->call()->name(), the same issue occurs when scheduling a job. Please see issue #29394 for more information including the steps to reproduce on a fresh Laravel install.

The origin of the problem

The PHP Fatal errors are thrown when removing the mutex of a CallbackEvent. (see source)

protected function removeMutex()
{
    if ($this->description) {
        $this->mutex->forget($this);
    }
}

As you can see we only remove the mutex if a description was provided. However, since we are not using the withoutOverlapping option, that mutex was never created in the first place. (see source)

if ($this->description && $this->withoutOverlapping &&
    ! $this->mutex->create($this)) {
    return;
}

The solution

Making sure that the conditions of creating and removing an event mutex are identical fixes our issue.

protected function removeMutex()
{
    if ($this->description && $this->withoutOverlapping) {
        $this->mutex->forget($this);
    }
}

Fix #29394