fix implem

Author: Vincent Molinié

src/parse-result.ts

@@ -1109,7 +1109,6 @@ export default class ParseResult {
         break;
       case 'BooleanLiteral':
       case 'NumberLiteral':
-      case 'NullLiteral':
         {
           let { source } = nodeInfo;
 
@@ -1121,6 +1120,11 @@ export default class ParseResult {
           output.push(source);
         }
         break;
+      case 'NullLiteral':
+        {
+          output.push('null');
+        }
+        break;
       default:
         throw new Error(
           `ember-template-recast does not have the ability to update ${original.type}. Please open an issue so we can add support.`