Fixed #11675 ValueToDuplicatesTransformer accept "0" value

Fixed wrong return null syntax

Added transformation to null on empty arrays

Removed useless statement in condition and switched to yoda condition

src/Symfony/Component/Form/Extension/Core/DataTransformer/ValueToDuplicatesTransformer.php

@@ -64,7 +64,7 @@ class ValueToDuplicatesTransformer implements DataTransformerInterface
         $emptyKeys = array();
 
         foreach ($this->keys as $key) {
-            if (!empty($array[$key])) {
+            if (isset($array[$key]) && '' !== $array[$key] && false !== $array[$key] && array() !== $array[$key]) {
                 if ($array[$key] !== $result) {
                     throw new TransformationFailedException(
                         'All values in the array should be the same'

src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/ValueToDuplicatesTransformerTest.php

@@ -82,6 +82,29 @@ class ValueToDuplicatesTransformerTest extends \PHPUnit_Framework_TestCase
         $this->assertNull($this->transformer->reverseTransform($input));
     }
 
+    public function testReverseTransformEmptyArray()
+    {
+        $input = array(
+            'a' => array(),
+            'b' => array(),
+            'c' => array(),
+        );
+
+        $this->assertNull($this->transformer->reverseTransform($input));
+    }
+
+
+    public function testReverseTransformZeroString()
+    {
+        $input = array(
+            'a' => '0',
+            'b' => '0',
+            'c' => '0'
+        );
+
+        $this->assertSame('0', $this->transformer->reverseTransform($input));
+    }
+
     /**
      * @expectedException \Symfony\Component\Form\Exception\TransformationFailedException
      */