fix: add support for null response fields (twilio#115)

Author: eshanholtz

src/services/twilio-api/twilio_accounts_v1.json

@@ -93,25 +93,30 @@
       },
       "date_time_iso8601": {
         "format": "date-time",
+        "nullable": true,
         "type": "string"
       },
       "sid_AC": {
         "maxLength": 34,
         "minLength": 34,
+        "nullable": true,
         "pattern": "^AC[0-9a-fA-F]{32}$",
         "type": "string"
       },
       "sid_CR": {
         "maxLength": 34,
         "minLength": 34,
+        "nullable": true,
         "pattern": "^CR[0-9a-fA-F]{32}$",
         "type": "string"
       },
       "string": {
+        "nullable": true,
         "type": "string"
       },
       "url": {
         "format": "uri",
+        "nullable": true,
         "type": "string"
       }
     },