Update OAS for Preview

Author: update-oas-bot

api-specifications/marketingsolutions_preview.json

@@ -11188,7 +11188,8 @@
           },
           "headlineFont": {
             "type": "string",
-            "description": "Font of the headline\r\nValid supported font like \"Arial\""
+            "description": "Font of the headline\r\nValid supported font like \"Arial\"",
+            "nullable": true
           },
           "headlineText": {
             "type": "string",