[Validator] Fixed LegacyValidator when only a constraint is validated

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The 2.5-BC API is currently broken for the following use case:

```php
$validator->validate(null, new NotNull());
```

This is fixed now.