Hi,
Empty values of custom product fields for the default variant should also not be displayed, unless you're using an old version of HikaShop.
The layout of the page in constructed with the default variant data in the show_default view file, which loads all the different sub view files like show_block_custom_main
Then, all the variants data's HTML is added hidde on the page via the show view file so that it can be swapped when you change the selection of the characteristics.
That way, you can have different prices, images, quantities, custom product fields, etc displayed on the product page and that it updates dynamically and instantaneously when you select another variant.