Fix errors on 20X responses with no body

`''` is not json-parsable, so  in that case we return null. This was the behavior prior to bac0e594ee

js/src/common/Application.tsx

@@ -438,6 +438,10 @@ export default class Application {
       }
 
       try {
+        if (responseText === '') {
+          return null;
+        }
+
         return JSON.parse(responseText);
       } catch (e) {
         throw new RequestError<ResponseType>(500, `${responseText}`, options, xhr);