There isn't a way to automatically scale around the center, so to
accomplish this you need to animate the position of the clip at the
same time as the scale. Do this by adding timed values for the
"Position in track" attribute if you're scaling a single clip. If
you're scaling the whole track it's called "Track origin". When the
scale is zero you'd put the position at the center.
Please let me know if you need more help.