A slot has an attachment field which holds the current attachment. And you can set it to null.
For setting an attachment null in java, you'd go slot.setAttachment(null);
spine-runtimes/Slot.java at 3.6
However, if any of your animations key that slot to have a specific attachment, that attachment will get applied to that slot when the animation is applied through AnimationState.apply.
So if you want to override that, your call to setAttachment should be made after AnimationState.apply
The alternate way to do outfits where a slot is null is to use a skin, and have that skin not have an entry for that slot.