[Validator] Disallow empty file in FileValidator

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11125)
| Applies to    | 2.6+
What do you think about expanding just a little bit more:

>
This constraint checks to see if the uploaded file is empty (i.e. 0 bytes). If it is, this message is displayed.