[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.