FIX: Fix CompositeField test that relied on a DropdownField bug

A DropdownField without a source no longer returns a validation
error.

tests/php/Forms/CompositeFieldTest.php

@@ -93,7 +93,7 @@ class CompositeFieldTest extends SapphireTest
     public function testValidation()
     {
         $field = CompositeField::create(
-            $fieldOne = DropdownField::create('A'),
+            $fieldOne = DropdownField::create('A', '', [ 'value' => 'value' ]),
             $fieldTwo = TextField::create('B')
         );
         $validator = new RequiredFields();