Yes, you're correct, now there is no easy option to set the AssignedTo variable in a ticket to empty from the Flow. It's due to the fact that the Assigned To is a Person or Group column and there is no easy way to set it to an empty value from Power Automate as of today.
Please check this post on the MS forum for a current solution using the default SharePoint "Update item" action. However, it seems that it works for multiple value Person or Group field only.
I will discuss it with our team and we'll see what we can do about this specific case. Thank you for the info!