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 implements the [Server
Optimization](https://xmpp.org/extensions/xep-0410.html#serveroptimization)
part of [XEP-0410: MUC Self-Ping]

# Usage

The module is loaded on MUC components:

```lua
Component "muc.example.com" "muc"
modules_enabled = {
    "muc_ping";
}
```

# Configuration

No options.

# Compatibility

It should work with Prosody up until 0.10.x.

Prosody 0.11.x and trunk natively supports XEP-0410 so this module is **not** needed.