Extra item in resulting Array

Hi, I was testing this out and I noticed an extra item is included in the resulting Array:

```
$ node -v
v8.6.0
$ npm install array.prototype.flatmap
+ array.prototype.flatmap@1.1.0
$ node -e "console.log(require('array.prototype.flatmap')([1, 2, 3], a => [a]))"
[ 1, 2, 3, <1 empty item> ]
```