Removed the use of $this->getHelperSet() as it is null by default

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12805
| License       | MIT
| Doc PR        | ~

Before:
![capture d ecran 2014-12-02 a 15 51 38](https://cloud.githubusercontent.com/assets/667519/5264798/303bf87a-7a3e-11e4-9b45-68e8a86bab60.png)

After:
![capture d ecran 2014-12-02 a 16 36 25](https://cloud.githubusercontent.com/assets/667519/5265299/b13190d6-7a41-11e4-9e68-d889ea290cae.png)

You could check first if a `HelperSet` and then the `FormatterHelper` can be retrieved and use this line only as a fallback.