Revert "[Form] Remove "value" attribute on empty_value option"

This reverts commit 9e849eb78bcc668a2b8aad39cb47e2ad32c73a47.

Reasons for the revert:

* https://github.com/symfony/symfony/issues/8478
* https://github.com/symfony/symfony/issues/8526
merged branch bschussek/revert-7678 (PR #8623)

This PR was merged into the 2.3 branch.

Discussion
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Revert "[Form] Remove "value" attribute on empty_value option"

| Q             | A
| ------------- | ---
| Bug fix?      | yes (together with #8622)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7678 #8478 #8526
| License       | MIT
| Doc PR        | -

Commits
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dbd14fc Revert "[Form] Remove "value" attribute on empty_value option"