avoid TypeError on when registering service workers without config

Fixes facebook#5299

Author: peterbe

Diff for: packages/react-scripts/template/src/serviceWorker.js

@@ -78,7 +78,7 @@ function registerValidSW(swUrl, config) {
               console.log('Content is cached for offline use.');
 
               // Execute callback
-              if (config.onSuccess) {
+              if (config && config.onSuccess) {
                 config.onSuccess(registration);
               }
             }