just use `if (typeof callback === 'function')`

I had a look in this code and you need also to replace
```javascript
    navigator.mozContacts.addEventListener(
      'contactchange',
      this.updateHeaderData
    );
```
by
```javascript
    navigator.mozContacts.addEventListener(
      'contactchange',
      this.updateHeaderData.bind(this)
    );
```
Otherwise "this" is undefined in the function and I think all lines with `this` fail. _that_ is a regression ;)