[Form][2.3] Fixes empty file-inputs getting treated as extra field.
bug #10251 [Form][2.3] Fixes empty file-inputs getting treated as extra field. (jenkoian)

This PR was squashed before being merged into the 2.3 branch (closes #10251).

Discussion
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[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)

Commits
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8d99d75 [Form][2.3] Fixes empty file-inputs getting treated as extra field.
Merge branch '2.3' into 2.4

* 2.3:
  [Form][2.3] Fixes empty file-inputs getting treated as extra field.
Merge branch '2.4'

* 2.4:
  [Form][2.3] Fixes empty file-inputs getting treated as extra field.
  changed some PHPUnit assertions to more specific ones
  fixed Kernel::stripComments() normalizing new-lines
  added a BC comment
  Update FileLoader to fix issue #10339
  bumped Symfony version to 2.3.12
  updated VERSION for 2.3.11
  update CONTRIBUTORS for 2.3.11
  updated CHANGELOG for 2.3.11
  Throw exception when unable to normalize embedded object
  Fixed evaluation of short circuit operators
  Follow-up to #10312: Fixed minor performance related issues in Yaml\Inline.
  [2.4][HttpKernel] Fix issue #10209 When the profiler has `only_exception` option activated and a subrequest throw an exception, the parent profile cannot be found.