EllipseGeometryUpdater crashes when positions is defined but positions.getValue(time) is undefined

reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/6qQ9NzKNzWk

A `DeveloperError` is being thrown from `EllipseGeometry` because the center position is undefined

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');

viewer.entities.add({
position : new Cesium.SampledPositionProperty(),
ellipse : {
semiMinorAxis : 3000.0,
semiMajorAxis : 3000.0,
material : Cesium.Color.GREEN
}
});
```

Start by locating EllipseGeometryUpdater and reproduce the provided entity example with a SampledPositionProperty whose getValue(time) is undefined. Trace how the undefined center reaches EllipseGeometry, then verify that the example no longer throws a DeveloperError while valid ellipse positions continue to work.

Written by the indexing model from the issue text.