When sending jabber:iq:last queries without 'to' attribute the server should not return its uptime, but instead handle the query on behalf of the user and therefore should return 0:

If the user has at least one connected or available resource when the server receives the request, the response MUST (subject to local security policies) contain an empty <query/> element whose 'seconds' attribute is set to a value of '0'.

See also RFC 6120 § 10.3.3.