mod_http_xep227: Handle nil/errors opening archive stores This matches how mod_mam opens archive stores. nil can be returned when there is no data in the store yet (it doesn't exist).

This module blocks configured queries that change server state.

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

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