mod_csi_muc_priorities: Fix bad argument to 'ipairs' on a missing list If either of the two list-multi are empty then apparently nothing thougth to include an empty array. Meh.

This module blocks configured queries that change server state.

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

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