an `undefined` value prevents merging (when merging 3+ objects)

[RebeccaStevens]

Bug Report

If one or more values are undefined, the mergeOthers route is taken.

Expected behavior

const x = { a: { b: 1 } };
const y = { a: undefined };
const z = { a: { c: 3 } };

const merged = deepmerge(x, y, z); // { a: { b: 1, c: 3 } }

Actual behavior

const x = { a: { b: 1 } };
const y = { a: undefined };
const z = { a: { c: 3 } };

const merged = deepmerge(x, y, z); // { a: z.a }