`dset` creates array instead of object for nested empty string property

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This test currently fails:
```js
objects(`should ${verb} empty string property`, () => {
let { input } = prepare({});

dset(input, ["hello", ""], 123);

assert.equal(input, {
hello: { "": 123 },
});
});
```

with
```
FAIL objects "should merge empty string property"
Expected values to be deeply equal: (equal)

··{
Actual:
--··"hello":·[]
Expected:
++··"hello":·{
++····"":·123
++··}
··}
```

Start by running the failing objects test shown in the issue and inspect the dset implementation it exercises. The work is done when setting the path ["hello", ""] produces an object containing the empty-string property with value 123, and the test passes.

Written by the indexing model from the issue text.