Add missing null check in QAbstractButtonPrivate::queryButtonList()

QCheckBox cb;
cb.setChecked(true);
cb.setChecked(false); 

 QList<QAbstractButton *>QAbstractButtonPrivate::queryButtonList() const  {
 #if QT_CONFIG(buttongroup)
 if (group)
     return group->d_func()->buttonList;
 #endif

+if (!parent) return {};
+
 QList<QAbstractButton*>candidates = parent->findChildren<QAbstractButton *>();
 if (autoExclusive) {
     auto isNoMemberOfMyAutoExclusiveGroup = [](QAbstractButton *candidate) {
         return !candidate->autoExclusive()
 #if QT_CONFIG(buttongroup)