fix(prosody) fix username not being set in mod_auth_jitsi-anonymous

If the session was not resumed the plugin must generate a random username, just
like the regular anonymous authentication plugin does.

resources/prosody-plugins/mod_auth_jitsi-anonymous.lua

 -- * session resumption
 -- Copyright (C) 2021-present 8x8, Inc.
 
+local generate_random_id = require "util.id".medium;
 local new_sasl = require "util.sasl".new;
 local sasl = require "util.sasl";
 local sessions = prosody.full_sessions;
 module:provides("auth", provider);
 
 local function anonymous(self, message)
+    -- Same as the vanilla anonymous auth plugin
+    local username = generate_random_id():lower();
+
     -- This calls the handler created in 'provider.get_sasl_handler(session)'
     local result, err, msg = self.profile.anonymous(self, username, self.realm);
 
     if result == true then
+        if (self.username == nil) then
+            -- Session was not resumed
+            self.username = username;
+        end
         return "success";
     else
         return "failure", err, msg;