fix handling null as empty data

Author: xabbuh

Extension/Core/DataTransformer/NumberToLocalizedStringTransformer.php

@@ -143,11 +143,11 @@ public function transform($value)
      */
     public function reverseTransform($value)
     {
-        if (!\is_string($value)) {
+        if (null !== $value && !\is_string($value)) {
             throw new TransformationFailedException('Expected a string.');
         }
 
-        if ('' === $value) {
+        if (null === $value || '' === $value) {
             return null;
         }
 

Tests/Extension/Core/Type/NumberTypeTest.php

@@ -85,4 +85,19 @@ public function testSubmitNullUsesDefaultEmptyData($emptyData = '10', $expectedD
         $this->assertSame($expectedData, $form->getNormData());
         $this->assertSame($expectedData, $form->getData());
     }
+
+    public function testSubmitNullWithEmptyDataSetToNull()
+    {
+        $form = $this->factory->create(static::TESTED_TYPE, null, [
+            'empty_data' => null,
+        ]);
+        $form->submit(null);
+
+        $this->assertTrue($form->isSubmitted());
+        $this->assertTrue($form->isSynchronized());
+        $this->assertTrue($form->isValid());
+        $this->assertSame('', $form->getViewData());
+        $this->assertNull($form->getNormData());
+        $this->assertNull($form->getData());
+    }
 }