immutable.set by index should generate `undefined` instead of empty

[syabro]

const immutable = require('object-path-immutable')
const x = {}
console.log(immutable.set(x,'arr.5', 'hello'))

returns
{ arr: [ <5 empty items>, 'hello' ] }

Which leads to different bugs, for example x.arr.map will iterate only 'hello' element.