Fixed empty file validation at form edit.

Added the file back into the form data (lost onbind)  for NotNull validation.

If I want to make a file mandatory:

```
    /**
     * @Assert\NotNull
     * @Assert\File(
     *     mimeTypes={"image/png", "image/jpeg", "image/pjpeg", "image/gif"}
     * )
     * @Vich\UploadableField(mapping="page_item", fileNameProperty="imagePath")
     */
    protected $image;
```

It works as expected when making a new object but it fails on edit because, as the file is not uploaded again, the file property is empty at validation time.

That's why I added the file back in the form's data if it's not being deleted or if it's not being changed (if changed the file will be on the newly binded data).