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.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```