Disable error message on a single rule in form validation

That kinda what you asked, you only want it to validate if it's not empty. So remove "required" and add "permit_empty".

If it's possible to disable it, it would be by passing "null" or just an empty string for said message.

It's not trimmed, but that rule dosen't trim it, at least not for when you are saving it into the database. It dosen't touch your original data.