Fix json encoded attribute backend type when attribute value is null

[tkotosz]

When you create a new product attribute with JsonEncoded backend type then you always get the following error when you try to load a product which doesn't have value for that specific attribute yet:

Exception #0 (Magento\Eav\Model\Entity\Attribute\Exception): Unable to unserialize value.

The source of the problem is this method:

public function unserialize($string)
    {
        $result = json_decode($string, true);
        if (json_last_error() !== JSON_ERROR_NONE) {
            throw new \InvalidArgumentException('Unable to unserialize value.');
        }
        return $result;
    }

When you do json_decode(null, true), then json_last_error() will return 4 (JSON_ERROR_SYNTAX).

With the suggested fix it will set the attribute value to empty array when the attribute value is null.

[ihor-sviziev]

@tkotosz could you cover your change with unit test in order to prevent regression in this part in future?

[dmanners]

Thanks for the PR @tkotosz I am happy with the code but I would love to see a test case to cover this. Currently the method testAfterLoad only covers the case when the attribute has been set.

[tkotosz]

@dmanners updated

[ihor-sviziev]

Could you move this new code as separate method (test case)?

[tkotosz]

@ihor-sviziev sure, done. Also I have squashed my commits.

[dmanners]

Add the visibility on the new test method please.

[dmanners]

guess you also need to add the visibility on this method.

[tkotosz]

@dmanners fix pushed :)