qt: fix crash when there is nothing to cache

On a fresh install of VLC compiler with mingw LLVM17 I get a crash when
accessing m_cachedData. The pointer is NULL because the queryCount is 0.

modules/gui/qt/util/listcache.hxx

@@ -479,6 +479,9 @@ void ListCache<T>::asyncCountAndLoad()
             size_t queryCount = (m_limit > 0)
                 ? std::min(m_limit, maximumCount)
                 : maximumCount;
+
+            if (queryCount == 0)
+                return;
             //note: should we drop items past queryCount?
             m_cachedData = std::make_unique<CacheData>(std::move(list),
                                                        queryCount,