ParticleSystem doesn't work with undefined lifetime

Nothing shows up until you uncomment `lifetime`

```js
var viewer = new Cesium.Viewer('cesiumContainer', {shouldAnimate: true});
var scene = viewer.scene;

var pos1 = Cesium.Cartesian3.fromDegrees(-75.15787310614596, 39.97862668312678);
var particleSystem = scene.primitives.add(new Cesium.ParticleSystem({
image : '../../../../Apps/SampleData/fire.png',
emissionRate: 5.0,
emitter: new Cesium.SphereEmitter(5.0),
imageSize : new Cesium.Cartesian2(25.0, 25.0),
modelMatrix : Cesium.Transforms.eastNorthUpToFixedFrame(pos1),
//lifetime : 16
}));
scene.camera.setView({destination: pos1});
```

Start by reproducing the supplied ParticleSystem example with lifetime omitted, then locate the ParticleSystem entry point and trace how an undefined lifetime is handled. Done means particles render in the provided setup without uncommenting lifetime, with regression coverage if an existing particle-system test location is found.

Written by the indexing model from the issue text.