Somehow I missed when we decided to allow null/undefined as the iterable
value in for-of loops.

The following test passes using the spec algorithms:

var c = 0;
for (var x of null) {
  c++;
}
assert.equal(c, 0);

However, if we get a null value here we are most likely just masking an
user bug.

I assume the justification is that for-in allows null here? However, for-of
is new syntax and we have the chance to get this right this time around.
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Somehow I missed when we decided to allow null/undefined as the iterable
value in for-of loops.

The following test passes using the spec algorithms:

```js
var c = 0;
for (var x of null) {
  c++;
}
assert.equal(c, 0);
```

However, if we get a null value here we are most likely just masking an
user bug.

I assume the justification is that for-in allows null here? However, for-of
is new syntax and we have the chance to get this right this time around.