🌊 Streams: Fix redirect bug (elastic#203640)

As discussed here:
elastic#202372 (comment)

There is a bug redirecting the user away from any management tab but
`route`. This was happening because while loading the definition it
would behave as if the stream is unmanaged. This fixes this problem.

x-pack/plugins/streams_app/public/components/stream_detail_management/index.tsx

@@ -80,7 +80,7 @@ export function StreamDetailManagement({
     );
   }
 
-  if (!definition?.managed) {
+  if (definition && !definition.managed) {
     return (
       <RedirectTo
         path="/{key}/management/{subtab}"