Prevent null reference access when unsetting styles

#### Repro

1. Set component `style` to a valid value.
2. Set component `style` to `undefined`.

#### Expected

- Styles from 1.) are unset.

#### Actual

- Error `Cannot read property 'hasOwnProperty' of undefined` is thrown.

http://jsfiddle.net/gasi/9rWum/

![create_a_new_fiddle_-_jsfiddle](https://cloud.githubusercontent.com/assets/49583/3236190/0802545e-f0db-11e3-8a3b-16ab183759ac.png)


- Signed CLA.
- Added tests.
- No new lint warnings.