Nodes: `while` with empty block is inconsistent with empty-block `for`

Nobody has claimed this yet.

- `for` with single-expression block containing `undefined`:

```
$ coffee -be 'console.log (for a in [0, 1, 2, 3] then undefined)'
[ undefined, undefined, undefined, undefined ]
```
- `for` with empty block:

```
$ coffee -be 'console.log (for a in [0, 1, 2, 3] then)'
[]
```
- `while` with single-expression block containing `undefined`:

```
$ coffee -be 'i = 4; console.log (while i-- then undefined)'
[ undefined, undefined, undefined, undefined ]
```
- `while` with empty block:

```
$ coffee -be 'i = 4; console.log (while i-- then)'
undefined
```

The value of a `for` or `while` should _always_ be a list. The behaviour of `while` with an empty block should change to produce an empty list so that it is consistent with `for`.

**edit:** an alternative (which I would actually prefer) would be to change the behaviour of the empty-block forms to match the forms with an `undefined` expression.

Start with the CoffeeScript compiler entry points that handle `for` and `while` expressions, using the reported `coffee -be` examples as reproduction cases. Add regression coverage for empty-block loops and verify that their results consistently follow the chosen behavior for blocks containing `undefined`.

Written by the indexing model from the issue text.