[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
Full sentence please. An empty file is not allowed. Also the problem that the file is opened in another program. Is it unique to Windows? Or is it the same bahavior in other OS and programs?