Revert "[Form] removed a constraint in PropertyPath as the path can definitely be an empty string for errors attached on the main form (when using a constraint defined with the 'validation_constraint' option)"

This reverts commit 5182a0c2c4.

PropertyPath instances should be empty. If you have an empty property path string, there is no need to create a PropertyPath instance for it.

Conflicts:

	tests/Symfony/Tests/Component/Form/PropertyPathTest.php

src/Symfony/Component/Form/Tests/Util/PropertyPathTest.php

@@ -440,6 +440,14 @@ class PropertyPathTest extends \PHPUnit_Framework_TestCase
         new PropertyPath('property.$form');
     }
 
+    /**
+     * @expectedException Symfony\Component\Form\Exception\InvalidPropertyPathException
+     */
+    public function testInvalidPropertyPath_empty()
+    {
+        new PropertyPath('');
+    }
+
     /**
      * @expectedException Symfony\Component\Form\Exception\InvalidPropertyPathException
      */

src/Symfony/Component/Form/Util/PropertyPath.php

@@ -77,7 +77,7 @@ class PropertyPath implements \IteratorAggregate
      */
     public function __construct($propertyPath)
     {
-        if (null === $propertyPath) {
+        if ('' === $propertyPath || null === $propertyPath) {
             throw new InvalidPropertyPathException('The property path must not be empty');
         }