Fix crash on accessing WebEngineView properties too early

WebContentsAdapter is not created together with the QQuickWebEngineView.
Thus querying a view property can lead to a crash if it uses the adapter.
This fix adds the missing guards for contentSize and scrollPosition
as it is done for similar WebEngineView properties.

Task-number: QTBUG-65942
Change-Id: I9c2668a059b08325629f5730608280ba7f3669cf
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/webengine/api/qquickwebengineview.cpp

@@ -1719,12 +1719,16 @@ void QQuickWebEngineView::triggerWebAction(WebAction action)
 QSizeF QQuickWebEngineView::contentsSize() const
 {
     Q_D(const QQuickWebEngineView);
+    if (!d->adapter)
+        return QSizeF();
     return d->adapter->lastContentsSize();
 }
 
 QPointF QQuickWebEngineView::scrollPosition() const
 {
     Q_D(const QQuickWebEngineView);
+    if (!d->adapter)
+        return QPointF();
     return d->adapter->lastScrollOffset();
 }