[Validator] Added allowEmpty option to File constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3746
I agree with @romainneutron this name is confusing. One could also rename the option to `disallowEmpty` to avoid the problem.