[5.5] Show null if a relationship is loaded but has value of null

[themsaid]

Let's say a Post belongs to an optional Author, so sometimes the author_id of a post is null.

Having the following in a resource:

"author" => AuthorResource::make($this->whenLoaded('author'))

In case author relationship is not loaded the author key won't appear in the response, in case the author is loaded and has value the AuthorResource will render the array format of the author.

However, in case the relationship is loaded but author_id = null, AuthorResource::make(null) will throw an exception.

This PR checks if any of the attributes holds a resource whose $resource attribute = null and just renders null to indicate that this post has no author.

"author" => null