Don't return a null QMimeData from the clipboard

The documentation for QClipboard::mimeData() doesn't say that the returned
value can be null, and some clients just dereference that without checking.
So instead return an empty QMimeData.

Change-Id: Ieec3140af4e7f33cde98ed96fd96b2674d0d0f9f
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@hawaiios.org>
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/plugins/platforms/wayland/qwaylandclipboard.cpp

@@ -54,11 +54,11 @@ QWaylandClipboard::~QWaylandClipboard()
 QMimeData *QWaylandClipboard::mimeData(QClipboard::Mode mode)
 {
     if (mode != QClipboard::Clipboard)
-        return 0;
+        return &m_emptyData;
 
     QWaylandInputDevice *inputDevice = mDisplay->currentInputDevice();
     if (!inputDevice || !inputDevice->dataDevice())
-        return 0;
+        return &m_emptyData;
 
     QWaylandDataSource *source = inputDevice->dataDevice()->selectionSource();
     if (source) {
@@ -68,7 +68,7 @@ QMimeData *QWaylandClipboard::mimeData(QClipboard::Mode mode)
     if (inputDevice->dataDevice()->selectionOffer())
         return inputDevice->dataDevice()->selectionOffer()->mimeData();
 
-    return 0;
+    return &m_emptyData;
 }
 
 void QWaylandClipboard::setMimeData(QMimeData *data, QClipboard::Mode mode)

src/plugins/platforms/wayland/qwaylandclipboard_p.h

@@ -47,6 +47,7 @@
 
 #include <qpa/qplatformclipboard.h>
 #include <QtCore/QVariant>
+#include <QtCore/QMimeData>
 
 #include <QtWaylandClient/private/qwaylandclientexport_p.h>
 
@@ -70,6 +71,7 @@ public:
 
 private:
     QWaylandDisplay *mDisplay;
+    QMimeData m_emptyData;
 };
 
 }