[Validator] Disallow empty file in FileValidator

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11125)
| Applies to    | 2.6+
I think we can also mention that the behavior has changed (not just a new option). Perhaps:

>
The ``disallowEmptyMessage`` option was introduced in Symfony 2.6. Before 2.6, 
if the user uploaded an empty, no validation error occurred.