mod_groups_internal: Fix traceback when room doesn't exist I'm not sure if it should even be included if it doesn't exist, but I'm not currently sure how this situation occurs, so I'm implementing the lightest possible fix for now.

This module blocks configured queries that change server state.

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

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