minor symfony#44888 [Config] Fix return type of ExprBuilder::ifEmpty() (gassan)

This PR was merged into the 4.4 branch.

Discussion
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[Config] Fix return type of `ExprBuilder::ifEmpty()`

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| License       | MIT
| Doc PR        | N/A

phpstan probably moans otherwise: Call to an undefined method Symfony\Component\Config\Definition\Builder\NodeParentInterface::scalarNode().

The following code part does not cause any errors:
```php
->ifTrue(function ($v) {return empty($v);})
```

Commits
-------

a525715 Update ExprBuilder.php

Author: fancyweb

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

@@ -91,7 +91,7 @@ public function ifNull()
     /**
      * Tests if the value is empty.
      *
-     * @return ExprBuilder
+     * @return $this
      */
     public function ifEmpty()
     {