FileField & ImageField: empty link on detail page

[Seb33300]

Describe the bug
When using an optional FileField or an ImageField in a crud, on the detail page, if no file/image was uploaded, an empty clickable link is generated returning a 404.
And for the image, an empty image is trying to be rendered:

To Reproduce

1. Create a crud with a nullable FileField or ImageField.
2. Save entity without file/image.
3. Open the detail page
4. You should see a clickable link to 404 error page