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