dset creates array instead of object for nested empty string property

[bgoscinski]

This test currently fails:

	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
        ++··}
        ··}