Fix validating key with null value

[KartaviK]

I found another bug:

If my enumeration contains a constant with the value "null", then the isValidKey() method incorrectly checks the validity:

<?php

final class MyEnum extends \MyCLabs\Enum\Enum
{
    public const UNDEFINED = null;
}

$valid = MyEnum::isValidKey('UNDEFINED'); // will return false because of `isset` checking

[mnapoli]

Thanks!