Missing Labels on Images throws exception

[mohan-nishit]

Hi Michiel, First of all, thank you for such a novel concept. I was spinning this up and connecting to one of our stores, and right off the bat, got an exception: Argument 2 passed to App\DTO\ProductImage::__construct() must be of the type string, null given

It was easy to figure out using the message that the Product DTO constructor enforces a datatype of string, but for the images where there was no label specified, the validation failed. I simply removed the validation on the constructor parameters to get this to work for the site in question without any ill effect.

I can send a PR for this if you would like.