What is the expected behavior of presence when the UUID is either not specified or its value is undefined?

A client which does not specify a UUID or includes a UUID with an undefined or null value will not be counted in presence statistics. In JavaScript, a unspecified UUID results in an auto-generated value which is then cached by the browser in local storage (or alternatively in a cookie). In general, it is best for an app to explicitly generate and cache the UUID value by the client.