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'