[Validator] YamlLoader: always check array presence

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | n/a

- Removes the useless `file_exists` check (already done by `is_file()` in the `FileLoader` constructor)
- Reset `$this->classes` to `null` before throwing an exception. Currently, the exception is not thrown the second time if `loadClassMetadata()` is called 2 times.