GroundPolylineGeometry should detect when only one unique point given.

[OmarShehata]

@likangning93 currently, the following code crashes with the following error.

var viewer = new Cesium.Viewer('cesiumContainer');
viewer.entities.add({
    polyline : {
        positions : [new Cesium.Cartesian3(1,1,1),new Cesium.Cartesian3(1,1,1)],
        clampToGround : true,
        width : 3
    }
});

An error occurred while rendering.  Rendering has stopped.
undefined
DeveloperError: normalized result is not a number
Error
    at new DeveloperError (http://localhost:8080/Source/Core/DeveloperError.js:43:19)
    at Function.Cartesian3.normalize (http://localhost:8080/Source/Core/Cartesian3.js:421:19)
    at direction (http://localhost:8080/Source/Core/GroundPolylineGeometry.js:334:20)
    at computeRightNormal (http://localhost:8080/Source/Core/GroundPolylineGeometry.js:188:23)
    at Function.GroundPolylineGeometry.createGeometry (http://localhost:8080/Source/Core/GroundPolylineGeometry.js:489:28)
    at http://localhost:8080/Source/Workers/createGroundPolylineGeometry.js:17:47
    at Promise.then (http://localhost:8080/Source/ThirdParty/when.js:196:34)
    at http://localhost:8080/Source/ThirdParty/when.js:297:13
    at processQueue (http://localhost:8080/Source/ThirdParty/when.js:647:4)
    at _resolve (http://localhost:8080/Source/ThirdParty/when.js:333:4)

This is because, given two duplicate points, GroundPolylineGeometry will remove the duplicate, but it still assumes there will be at least 2 points. I think it should detect when that happens and not draw anything (see discussion here #6693 (comment) )

[likangning93]

Thanks for the heads up @OmarShehata!