mod_roster_command: Fix subscription when the "user JID" is a bare domain. Do not attempt to update the roster when the user is bare domain (e.g. a component), since they don't have rosters and the attempt results in an error: $ prosodyctl mod_roster_command subscribe proxy.example.com contact@example.com xxxxxxxxxxFailed to execute command: Error: /usr/lib/prosody/core/rostermanager.lua:104: attempt to concatenate local 'username' (a nil value) stack traceback: /usr/lib/prosody/core/rostermanager.lua:104: in function 'load_roster' /usr/lib/prosody/core/rostermanager.lua:305: in function 'set_contact_pending_out' mod_roster_command.lua:44: in function 'subscribe'

This module publishes data from [Prosody's `internal` statistics][doc:statistics]
 into a PubSub node in [XEP-0039] format.

The node defaults to `stats` but can be changed with the option
`pubsub_stats_node`.

``` lua
Component "pubsub.example.com" "pubsub"
modules_enabled = {
    "pubsub_stats";
}
pubsub_stats_node = "statistics"
```