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'

Contributing
============

Guidelines for developers
-------------------------

-   Each module should be contained in a folder of its name (e.g.\
     `mod_ping/mod_ping.lua`)
-   Each module should have a `README.markdown` page in their folder
    with a description, usage, configuration and todo sections (feel
    free to copy an existing one as a template)
-   Commit messages should begin with the name of the plugin they are
    for (e.g. `mod_ping: Set correct namespace on pongs`)

Instructions on cloning the repository are at
<https://prosody.im/doc/installing_modules#prosody-modules>