…tionHelper::ask() (chalasr)

This PR was merged into the 2.7 branch.

Discussion
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[Console] Fix validation of empty values using SymfonyQuestionHelper::ask()

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  |no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

When using `QuestionHelper::ask()` it's allowed to return an empty value as answer, e.g:

```php
$helper = new QuestionHelper();
$question = new Question('foo', false);
$question->setValidator(function ($v) { return $v; });
$answer = $helper->ask($input, $output, $question);
```

Just typing `enter` for answering this question works, the value of `$answer` would be `false`.
But doing the same with `SymfonyQuestionHelper::ask()`:

```php
$helper = new SymfonyQuestionHelper();
$question = new Question('foo', false);
$question->setValidator(function ($v) { return $v; });
$answer = $helper->ask($input, $output, $question);
```

>  [ERROR] A value is required.

Same for `''` or `null`.
Here I kept the same check but used as default validator, if a validator is set and allows an empty value to be returned then it's ok.

Also I am not sure about if this default validator should be kept, imho we should be consistent with the default question helper, using the `SymfonyQuestionHelper` should only impact the output.

Diff best viewed [like this](https://github.com/symfony/symfony/pull/20141/files?w=1)
ping @kbond

Commits
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a8b910b [Console] Fix validation of null values using SymfonyStyle::ask()