Fix getProperty to properly handle false values

[AntoineGautier]

The function getProperty defined at https://github.com/lbl-srg/modelica-json/blob/master/lib/jsonquery.js#L8-L10 returns null if a property has a false value.

getProperty(['foo', 'foz'], { foo: { foz: false }}); // returns null

This ticket is to use the nullish coalescing operator as below so that the function returns false in this case.

const getProperty = (p, o) => p.reduce((xs, x) => xs?.[x] ?? null, o)