mod_invites_adhoc: Set non-nil defaults for options, fixes traceback (thanks Martin)
Fixes an error when attempting to call the :empty() method on the
default implicit default nil value.
module:set_global();
local traceback = require "util.debug".traceback;
require"util.signal".signal(module:get_option_string(module.name, "SIGUSR1"), function ()
module:log("info", "Received SIGUSR1, writing traceback");
local f = io.open(prosody.paths.data.."/traceback.txt", "a+");
f:write(traceback(), "\n");
f:close();
end);