add nullpointer check

[jbo023]

add a nullpointer check for updateClamping function.

This fixes a bug where a removed billboard can prevent changing of the terrainProvider.
This only occurs if the billboardcollection is a child of a primitiveCollection, which is set to show=false, so the billboard never gets cleaned up.

The following Sandcastle Code can produce the problem

var viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain()
});

var scene = viewer.scene;

var primitiveCollection = new Cesium.PrimitiveCollection();
var billboards = primitiveCollection.add(new Cesium.BillboardCollection({scene: scene}));
scene.primitives.add(primitiveCollection);
var billboard = billboards.add({
    position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883, 150000),
    image : '../images/Cesium_Logo_overlay.png',
    heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
});

Sandcastle.addToolbarButton( 'crash',function() {
    billboards.remove(billboard);
    primitiveCollection.show = false;                
    viewer.terrainProvider = new Cesium.EllipsoidTerrainProvider({});
});

[cesium-concierge]

Thanks for the pull request @jbo023!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[hpinkos]

Thanks for the PR @jbo023! I just had that one comment, then I'll get this merged for the June release.

[jbo023]

@hpinkos thanks for reviewing, i changed the code to use defined and rebased to the current master

[hpinkos]

Great, thanks @jbo023!

[hpinkos]

Upated CHANGES.md here: 74e286c