filter collection with "groupBy" when keys can be null / undefined.

https://github.com/a8m/angular-filter/blob/91ae1bfe75a4e747c2c52eab2065565aeb39c97e/src/_filter/collection/group-by.js#L36-L43

Because of memoization, special values such as `null` or `undefined` converted to their string equivalent `'null'` and `'undefined'`.

e.g. I am trying to have an `accordion-group` whose heading is the name of the group, however I cannot test for `null` or `undefined` explicitly.

```

...something with groupData...
```