Fix scrolling on empty scrollview.

This fixes https://github.com/kivy/kivy/issues/1852.

The issue was with scrolling on a empty scrollview. This code reproduced the issue:

    from kivy.app import runTouchApp
    from kivy.lang import Builder
    kv = """
    ScrollView
    """
    if __name__ == '__main__':
        runTouchApp(Builder.load_string(kv))

Which would crash before this pr.