[Form][2.3] Fixes empty file-inputs getting treated as extra field.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8575 (https://github.com/symfony/symfony/pull/8575#issuecomment-34867485)
| License       | MIT

Re-applies 968fe23 (PR #8575).

The test for this already exists, it was just this line that got overwritten by https://github.com/symfony/symfony/commit/eb9f76d5bad120c1d4b3b30c2d8c73bb9d5928e8#diff-ca5e25b47f3ecc94cd557946aeb486c6L542

To clarify, this is a PR into 2.3 branch - this already exists in 2.4 (and later from this PR: https://github.com/symfony/symfony/pull/9146)