Don't assert in QQuickItem::update

The method can actually be called from QML, so we can not
have an assert in that method. Instead simply return if the
item has no contents

Task-number: QTBUG-34060
Change-Id: Ib28ffa5c6c63fbec956abe25020010ed73a9cfa9
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>