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
Fixes link indices
[Form] Fixed: If a form is not present in a request, it is not automatically submitted
Revert "[Form] Fix of "PATCH'ed forms are never valid""

This reverts commit a2b15359d81563d52b62b4357407efa949a0706a.

Conflicts:
	src/Symfony/Component/Form/Form.php

The commit is reverted because it introduces a bug demonstrated by a currently failing test.
[Form] Fixed patched forms to be valid even if children are not submitted
merged branch bschussek/revert-7678 (PR #8623)

This PR was merged into the 2.3 branch.

Discussion
----------

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
-------

dbd14fc Revert "[Form] Remove "value" attribute on empty_value option"
merged branch langj/readme-link-fix (PR #8632)

This PR was merged into the 2.3 branch.

Discussion
----------

Fixes link indices

Fixes links in the symfony/intl README.md

Commits
-------

d1e5710 Fixes link indices
merged branch bschussek/issue8385 (PR #8636)

This PR was merged into the 2.3 branch.

Discussion
----------

[Form] Fixed: If a form is not present in a request, it is not automatically submitted

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8385
| License       | MIT
| Doc PR        | -

This PR changes the following behavior of `handleRequest()`:

Current behavior:

```php
$_POST = array('foo' => 'bar');
$form = $factory->createNamed('myform', 'mytype');

// "myform" is not present in the request
$form->handleRequest();

assert(true === $form->isSubmitted());
```

Behavior after PR:

```php
$_POST = array('foo' => 'bar');
$form = $factory->createNamed('myform', 'mytype');

// "myform" is not present in the request
$form->handleRequest();

assert(false === $form->isSubmitted());
```

As #8385 pointed out, the latter behavior is expected, so I consider the current behavior a bug.

Commits
-------

cb5e765 [Form] Fixed: If a form is not present in a request, it is not automatically submitted
merged branch bschussek/issue8111 (PR #8638)

This PR was merged into the 2.3 branch.

Discussion
----------

[Form] Fixed patched forms to be valid even if children are not submitted

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8111
| License       | MIT
| Doc PR        | -

#8362 was reverted because it introduces a failing test that is caused by a regression. This PR takes the alternative approach that

* unsubmitted fields in the PATCH request remain unsubmitted
* `isValid()` ignores unsubmitted children
* `mapFormsToData()` ignores unsubmitted children

In my opinion this is a more proper solution than #8362.

Commits
-------

85330a6 [Form] Fixed patched forms to be valid even if children are not submitted
50f201e Revert "[Form] Fix of "PATCH'ed forms are never valid""