[HttpKernel] fixed file uploads in functional tests when no file was selected

Allow user to submit a form with no file selected.
bug #10953 [HttpKernel] fixed file uploads in functional tests without file selected (realmfoo)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #10953).

Discussion
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[HttpKernel] fixed file uploads in functional tests without file selected

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
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6f56d87 [HttpKernel] fixed file uploads in functional tests when no file was selected