To support #2059, but I think these are the correct way to round out this function anyways.

Also now goes through an internal list_interleave(), which is a convenient form when you already have a list that also exposes an x_arg for improved error messaging.

Also fixes an issue where vec_interleave(1:2, NULL, 1:3) was reporting the wrong index in the error message due to compacting out the NULL too early. Since list_combine() itself can handle NULLs and elegantly drops them at the right time, we now just pass through to that with an empty integer() location index.