Support for OR conditions in rules expression

Do role expressions support an OR condition? In my case, I would like for a role to be applied if either the reserver is the current user or if there is no reservation set. I imagined it would look like the below but this does’nt work

  {
  "reservation.reserver": "%%user.id",
  "$or": {
    "reservation": {
      "$exists": false
    }
  }
}