Don't return undefined when an empty model is passed to makerjs.measure.modelExtents

[xavi-]

Currently makerjs.measure.modelExtents({ models: {} }) returns undefined which makes code prone to NPE's

This PR updates modelExtents to instead return an empty IMeasureWithCenter object: { height: 0, width: 0, center: null, low: null, high: null }

[xavi-]

It might be better to return { width: 0, height: 0, center: [0, 0], low: [0, 0], high: [0, 0] } instead since according to the types, center, low, and high aren't nullable

[danmarshall]

Thanks for the PR!
But, since the whole library doesn't have 100% test coverage (my fault not yours), I'm a little hesitant to take this. Perhaps you could add your own defensive function in your own code:

function alwaysMeasureModel(model) {
   return makerjs.measure.modelExtents(model) || { width: 0, height: 0, center: [0, 0], low: [0, 0], high: [0, 0] };
}

[xavi-]

Or perhaps the return type of modelExtents should be changed to IMeasureWithCenter | null? I created this PR mostly because I got bitten by this in production

[danmarshall]

Sorry it bit you in production. Yes the | null is a good approach.