Ellipsoid with czml not rendering image uri properly

I am trying to render an ellipsoid with czml with an image uri. The image renders correctly when the position is not over time, but renders the default white when it is over time.

**Correct image rendered**
```
"position": {
"cartesian": [137633909.2680,349792303.3080,124291199.2880]
}
```

**Renders white ellipsoid without image**
```
"position": {
"epoch":"2014-07-22T11:29:11Z",
"cartesian": [0, 137633909.2680,349792303.3080,124291199.2880]
}
```

Sandcastle example: [Link](https://sandcastle.cesium.com/#c=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)

Browser: Chrome

Operating System: MacOS

Start by running the linked Sandcastle example in Chrome and compare the static position with the epoch-based, time-varying position. Trace the CZML ellipsoid image URI handling for both cases. Done means the supplied image renders instead of the default white ellipsoid when position is defined over time.

Written by the indexing model from the issue text.