Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant)

Qt-only changes.

src/qt/clientmodel.cpp

@@ -35,9 +35,9 @@ ClientModel::ClientModel(interfaces::Node& node, OptionsModel *_optionsModel, QO
     QObject(parent),
     m_node(node),
     optionsModel(_optionsModel),
-    peerTableModel(0),
-    banTableModel(0),
-    pollTimer(0)
+    peerTableModel(nullptr),
+    banTableModel(nullptr),
+    pollTimer(nullptr)
 {
     cachedBestHeaderHeight = -1;
     cachedBestHeaderTime = -1;