Remove Q_NULLPTR usage

This project requires C++17, which has the nullptr keyword.

src/qwaylandlayershellintegration.cpp

@@ -15,7 +15,6 @@
 namespace LayerShellQt
 {
 QWaylandLayerShellIntegration::QWaylandLayerShellIntegration()
-    : m_layerShell(Q_NULLPTR)
 {
 }
 

src/qwaylandlayershellintegration_p.h

@@ -28,7 +28,7 @@ public:
 private:
     static void registryLayer(void *data, struct wl_registry *registry, uint32_t id, const QString &interface, uint32_t version);
 
-    QWaylandLayerShell *m_layerShell;
+    QWaylandLayerShell *m_layerShell = nullptr;
 };
 
 }