[Serializer] fixed some tests that were failing on the CI server

tests/Symfony/Tests/Component/Serializer/Encoder/XmlEncoderTest.php

@@ -65,7 +65,7 @@ class XmlEncoderTest extends \PHPUnit_Framework_TestCase
                 'Bar' => "Test",
                 '@Type' => 'test'
             ),
-            'föo_bär' => '',
+            'föo_bär' => 'a',
             "Bar" => array(1,2,3),
             'a' => 'b',
         );
@@ -73,7 +73,7 @@ class XmlEncoderTest extends \PHPUnit_Framework_TestCase
             '<response>'.
             '<foo-bar id="1" name="Bar"/>'.
             '<Foo Type="test"><Bar><![CDATA[Test]]></Bar></Foo>'.
-            '<föo_bär><![CDATA[]]></föo_bär>'.
+            '<föo_bär><![CDATA[a]]></föo_bär>'.
             '<Bar>1</Bar>'.
             '<Bar>2</Bar>'.
             '<Bar>3</Bar>'.
@@ -86,16 +86,16 @@ class XmlEncoderTest extends \PHPUnit_Framework_TestCase
     {
         $obj = new ScalarDummy;
         $obj->xmlFoo = array(
-            'foo-bar' => '',
-            'foo_bar' => '',
-            'föo_bär' => '',
+            'foo-bar' => 'a',
+            'foo_bar' => 'a',
+            'föo_bär' => 'a',
         );
 
         $expected = '<?xml version="1.0"?>'."\n".
             '<response>'.
-            '<foo-bar><![CDATA[]]></foo-bar>'.
-            '<foo_bar><![CDATA[]]></foo_bar>'.
-            '<föo_bär><![CDATA[]]></föo_bär>'.
+            '<foo-bar><![CDATA[a]]></foo-bar>'.
+            '<foo_bar><![CDATA[a]]></foo_bar>'.
+            '<föo_bär><![CDATA[a]]></föo_bär>'.
             '</response>'."\n";
 
         $this->assertEquals($expected, $this->encoder->encode($obj, 'xml'));