fix($parse): return 'undefined' if a middle key's value is null

Prior to this fix, $parse/$eval would return 'null' if a middle key in an expression's value is null, when it should be expected to be undefined.

This patch tries to remedy this by returning undefined for middle values in  expressions, when fetching a child of that null value.

For example:

```js
// Given the following object:
$scope.a = {
 b: null
};

// $scope.$eval('a.b.c') returns undefined, whereas previously it would return null
```

Fixes #2249
Fixes #5442