`select1()` will return undefined when it can't find anything

[YbJerry]

When select1() can't find anything, it will return undefined. But at first, I think it should return null as I don't find undefined in type SelectSingleReturnType

In line 5000 of path.js

result = result.nodes;
if (single) {
    result = result[0];
}

In this situation, nodes will be an empty array. So result[0] will be undefined.

So I suggest that there should be a comment for users to know it will return undefined. Thanks.