[5.4] Model event result prevents firing observable events

[thijskok]

When a model uses the $event property as well as observable events (e.g., through a 3rd party package), sometimes event handling will stop before reaching the observables. This is because the model's event result is treated as "failed" in Illuminate/Database/Eloquent/Concerns/HasEvents.php, while it isn't:

$result = $this->fireCustomModelEvent($event, $method);

// ...
// For example, one event listener running successfully 
// causes the $result variable to be array(0 => null)
// ...

return ! empty($result) ? $result : static::$dispatcher->{$method}(
    "eloquent.{$event}: ".static::class, $this
);

Empty evaluates the array containing null as false, preventing the dispatcher from firing any subsequent events. The code fragment in this PR strips out the null elements, causing empty to be true when model events were handled successfully.