Prevents a lazy deopt in V8 when accessing `undefined` property

Quick summary:

V8's deoptimized `concatenatedMixinProperties` function that is called
by `mergeMixins`. `mergeMixins` is called by `applyMixin`.
Result was that inlined `concatenatedMixinProperties` function was deoptimized
and that caused deoptimization of `mergeMixin` and `applyMixin`.

`EMPTY_ARRAY` was added so the function signature remains monomorphic.

[Relevant V8
output](https://gist.github.com/twokul/d5f544a23213111f48cd#file-code-asm-L500-L506).