Don't allow empty searches in debug mode

They are not useful

libdiscover/resources/AbstractResourcesBackend.h

@@ -101,6 +101,8 @@ class DISCOVERCOMMON_EXPORT AbstractResourcesBackend : public QObject
             QString origin;
             bool allBackends = false;
 
+            bool isEmpty() const { return !category && state == AbstractResource::Broken && mimetype.isEmpty() && search.isEmpty() && extends.isEmpty() && resourceUrl.isEmpty() && origin.isEmpty(); }
+
             bool shouldFilter(AbstractResource* res) const;
             void filterJustInCase(QVector<AbstractResource*>& input) const;
         };

libdiscover/resources/ResourcesModel.cpp

@@ -322,6 +322,8 @@ AggregatedResultsStream * ResourcesModel::findResourceByPackageName(const QUrl&
 
 AggregatedResultsStream* ResourcesModel::search(const AbstractResourcesBackend::Filters& search)
 {
+    Q_ASSERT(!search.isEmpty());
+
     auto streams = kTransform<QSet<ResultsStream*>>(m_backends, [search](AbstractResourcesBackend* backend){ return backend->search(search); });
     return new AggregatedResultsStream(streams);
 }