fix(xmpp): show message for empty disco#items results (XEP-0030)

Per XEP-0030 section 3.1: 'if an entity has no associated items,
it MUST return an empty <query/> element.'

The client should display 'No service discovery items for X' when
receiving an empty result, not silently ignore it.

Previous version introduced in commit f28655c5c (2016-05-08) which added an early
return when child == NULL, preventing the message from being shown.

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);