Fix spec for Enum.chunks/4

Resolves the following dialyzer warnings from #1569:

```
lib/elixir/lib/enum.ex:1299: Function chunks/2 has no local return
lib/elixir/lib/enum.ex:1299: The call 'Elixir.Enum':chunks(coll::any(),n::any(),n::any(),'nil') breaks the contract (t(),non_neg_integer(),non_neg_integer(),[any()]) -> [[any()]]
lib/elixir/lib/enum.ex:1325: Function chunks/3 has no local return
lib/elixir/lib/enum.ex:1325: The call 'Elixir.Enum':chunks(_@D0::any(),_@D1::any(),_@D2::any(),'nil') breaks the contract (t(),non_neg_integer(),non_neg_integer(),[any()]) -> [[any()]]
```