Stop using null in fixture

Now that field is not nullable.

sites/_fixtures/promotions.json

@@ -45,7 +45,7 @@
             "body": "Welcome to Oscar's sandbox site.&nbsp; This is a example install of Oscar, making very few customisations to the core.&nbsp; It is intended as a playground for experimenting with Oscar's features.<br><br>You can get access to the dashboard <a href=\"http://latest.oscarcommerce.com/gateway/\" rel=\"nofollow\">using this form</a>.", 
             "date_created": "2013-01-08T17:12:09.526Z", 
             "name": "Welcome!", 
-            "display_type": null
+            "display_type": ""
         }
     }, 
     {