How can I save an unpublished node skipping required fields validation?
I finally solved this using the Require on Publish module. It offers an additional checkbox to require only when publishing the node.

This module allows fields to be required only when the piece of
content is being published or is already published. This can be used
when you have fields such as tags or SEO information on your piece of
content that editors really don't need to fill in until the content is
going live.