MAGETWO-36625: Extended attributes should be optional

    - Fix unit test failure

Author: Yu Tang

lib/internal/Magento/Framework/Api/Test/Unit/Code/Generator/_files/SampleExtensionInterface.txt

@@ -6,7 +6,7 @@ namespace Magento\Catalog\Api\Data;
 interface ProductExtensionInterface extends \Magento\Framework\Api\ExtensionAttributesInterface
 {
     /**
-     * @return string
+     * @return string|null
      */
     public function getStringAttribute();
 
@@ -17,7 +17,7 @@ interface ProductExtensionInterface extends \Magento\Framework\Api\ExtensionAttr
     public function setStringAttribute($stringAttribute);
 
     /**
-     * @return \Magento\Bundle\Api\Data\OptionInterface[]
+     * @return \Magento\Bundle\Api\Data\OptionInterface[]|null
      */
     public function getComplexObjectAttribute();