bug #20141 [Console] Fix validation of empty values using SymfonyQuestionHelper::ask() (chalasr)

This PR was merged into the 2.7 branch.

Discussion
----------

[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
-------

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

src/Symfony/Component/Console/Helper/SymfonyQuestionHelper.php

@@ -34,11 +34,11 @@ class SymfonyQuestionHelper extends QuestionHelper
         $question->setValidator(function ($value) use ($validator) {
             if (null !== $validator) {
                 $value = $validator($value);
-            }
-
-            // make required
-            if (!is_array($value) && !is_bool($value) && 0 === strlen($value)) {
-                throw new \Exception('A value is required.');
+            } else {
+                // make required
+                if (!is_array($value) && !is_bool($value) && 0 === strlen($value)) {
+                    throw new \Exception('A value is required.');
+                }
             }
 
             return $value;

src/Symfony/Component/Console/Tests/Helper/SymfonyQuestionHelperTest.php

@@ -6,6 +6,7 @@ use Symfony\Component\Console\Helper\FormatterHelper;
 use Symfony\Component\Console\Helper\HelperSet;
 use Symfony\Component\Console\Helper\SymfonyQuestionHelper;
 use Symfony\Component\Console\Output\StreamOutput;
+use Symfony\Component\Console\Question\Question;
 use Symfony\Component\Console\Question\ChoiceQuestion;
 
 /**
@@ -73,6 +74,15 @@ class SymfonyQuestionHelperTest extends \PHPUnit_Framework_TestCase
         $this->assertOutputContains('What is your favorite superhero? [Superman, Batman]', $output);
     }
 
+    public function testAskReturnsNullIfValidatorAllowsIt()
+    {
+        $questionHelper = new SymfonyQuestionHelper();
+        $questionHelper->setInputStream($this->getInputStream("\n"));
+        $question = new Question('What is your favorite superhero?');
+        $question->setValidator(function ($value) { return $value; });
+        $this->assertNull($questionHelper->ask($this->createInputInterfaceMock(), $this->createOutputInterface(), $question));
+    }
+
     protected function getInputStream($input)
     {
         $stream = fopen('php://memory', 'r+', false);