feature #19764 [Config] Add ExprBuilder::ifEmpty() (ogizanagi)

This PR was merged into the 3.2-dev branch.

Discussion
----------

[Config] Add ExprBuilder::ifEmpty()

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | symfony/symfony-docs#6922

Useful for instance when you don't expect to have a key set in the resolved config if its content is empty:

```php
$builder = new TreeBuilder();
$tree = $builder
    ->root('matcher')
        ->children()
            ->arrayNode('references_to_exclude')
                ->validate()
                    ->ifEmpty()
                    ->thenUnset()
                ->end()
                ->prototype('scalar')->end()
            ->end()
        ->end()
    ->end()
    ->buildTree()
;

$tree->finalize(['references_to_exclude' => ['foo', 'bar']]);
>>> ['references_to_exclude' => ['foo', 'bar']]

$tree->finalize(['references_to_exclude' => []]);
>>> []
```

Commits
-------

4e46f64 [Config] Add ExprBuilder::ifEmpty()

src/Symfony/Component/Config/Definition/Builder/ExprBuilder.php

@@ -97,6 +97,18 @@ public function ifNull()
         return $this;
     }
 
+    /**
+     * Tests if the value is empty.
+     *
+     * @return ExprBuilder
+     */
+    public function ifEmpty()
+    {
+        $this->ifPart = function ($v) { return empty($v); };
+
+        return $this;
+    }
+
     /**
      * Tests if the value is an array.
      *

src/Symfony/Component/Config/Tests/Definition/Builder/ExprBuilderTest.php

@@ -75,6 +75,21 @@ public function testIfNullExpression()
         $this->assertFinalizedValueIs('value', $test);
     }
 
+    public function testIfEmptyExpression()
+    {
+        $test = $this->getTestBuilder()
+            ->ifEmpty()
+            ->then($this->returnClosure('new_value'))
+        ->end();
+        $this->assertFinalizedValueIs('new_value', $test, array('key' => array()));
+
+        $test = $this->getTestBuilder()
+            ->ifEmpty()
+            ->then($this->returnClosure('new_value'))
+        ->end();
+        $this->assertFinalizedValueIs('value', $test);
+    }
+
     public function testIfArrayExpression()
     {
         $test = $this->getTestBuilder()