var/mob/x = input("Select a player to mute or unmute.") as null in o 
if(!x) return
x.muted = !x.muted

Problem description:

I'm getting a warning and I'm not sure why. Everything works as I want, but I don't want that warning: warning: it is recommended that you use an object filter or 'anything' when combining constant input types with lists

I had found archives of the question being asked, but those solutions didn't work.

You want it to be as null|anything in o, not just null.