[Validator] Disallow empty file in FileValidator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3746
Ok, if the problem with opened file had been in every OS, then we could added a hint to the validation message. But when it's not the case, then it's like this.