Views: use isNull for zero-size checks rather than isEmpty

QSizeF::isEmpty returns true if either dimension is negative, but
according to tst_QQuickListView::resizeView, we support negative-sized
views.

Amends 37c25c6.

Change-Id: If860cd5a459adf75470e5fc2deba3967b2e66366
Reviewed-by: Richard Moe Gustavsen <[email protected]>
(cherry picked from commit e646626)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>
(cherry picked from commit 4c0df79)
(cherry picked from commit f17ba18)

Author: mitchcurtis

src/quick/items/qquickgridview.cpp

@@ -2402,7 +2402,7 @@ bool QQuickGridViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
 {
     Q_Q(QQuickGridView);
 
-    if (q->size().isEmpty())
+    if (q->size().isNull())
         return false;
 
     int modelIndex = change.index;

src/quick/items/qquickitemview.cpp

@@ -1760,7 +1760,7 @@ void QQuickItemViewPrivate::refill(qreal from, qreal to)
     Q_Q(QQuickItemView);
     if (!model || !model->isValid() || !q->isComponentComplete())
         return;
-    if (q->size().isEmpty() && visibleItems.isEmpty())
+    if (q->size().isNull() && visibleItems.isEmpty())
         return;
     if (!model->count()) {
         updateHeader();

src/quick/items/qquicklistview.cpp

@@ -3658,7 +3658,7 @@ bool QQuickListViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
     int modelIndex = change.index;
     int count = change.count;
 
-    if (q->size().isEmpty() && visibleItems.isEmpty())
+    if (q->size().isNull() && visibleItems.isEmpty())
         return false;
 
     qreal tempPos = isContentFlowReversed() ? -position()-size() : position();