do not treat null/undefined as the empty list

Ideally one knows the types of one's values. When this is not the case, one can apply `Object` to the value in question. For example:

```javascript
var x = null;

R.head(x);
// => TypeError: Cannot read property '0' of null
R.head(Object(x));
// => undefined
```

I very much like the explicitness this change enforces. It is a breaking change, of course, but we're pre-1.0. ;)