fix: model not implementing has media should only throw exception if no custom callback is provided

[Tofandel]

Description

It's very much possible to implement an Image column with a custom thumbnail (eg coming from youtube or any other service)

Right now the following throws an exception if the Video model doesn't use HasMedias

            Image::make()->field('thumbnail')->title('Thumbnail')
                ->customRender(fn(Video $video) => $video->getThumbnail(80, 80))

But it shouldn't because of the custom render method provided

BTW I noticed a related issue on the browser, using any other field name than thumbnail on image in the browser columns results in the image being displayed as text instead of as an image as well as the order of the columns being wrong