Fix/achievement progress property

[ElmageAce]

I noticed this bug while working on a company project, we enabled laravel strict mode and realized there was a problem with a property.

Because we had strict mode enabled, this property unlockedAt mainly gave us problems. We eventually modified a fork of this but I thought it'd be great if I made a PR with the fix.

First time in a while I'm making PR to an open-source project, let me know if I missed anything. Thanks

public function isUnlocked(): bool
    {
        if (!is_null($this->unlockedAt)) {
            return true;
        }
        if ($this->points >= $this->details->points) {
            return true;
        }
        return false;
    }
`