How to show always a dropdown when products has attributes (Woocommerce)?

I have added variable products to my woocommerce site with variations (size) but some products has 1 size, how can i show always a dropdown when a product has variations? It doesnt matter how much.

For example

Product 1 - Medium (show dropdown)

Product 2 - Small, Medium, XL (Show dropdown)

For both scenario it shows a dropdown for choosing a size.