Add functional tests for /disco command (XEP-0030)

- Add 8 tests for disco info and disco items commands
- Fix XEP-0030 compliance bug: show message for empty disco#items results
- Tests cover: identity display, features, server/jid queries, error handling,
  items display, empty results, and connection requirement

src/xmpp/iq.c

@@ -2532,9 +2532,6 @@ _disco_items_result_handler(xmpp_stanza_t* const stanza)
     }
 
     xmpp_stanza_t* child = xmpp_stanza_get_children(query);
-    if (child == NULL) {
-        return;
-    }
 
     while (child) {
         const char* stanza_name = xmpp_stanza_get_name(child);