[ppr] Fix for error case involving prefetch data routes (#11638)

Only error when the prefetch data route is present but the route also
has PPR enabled.

packages/next/src/utils.ts

@@ -2270,7 +2270,7 @@ export const onPrerenderRoute =
       }
 
       outputPathPrefetchData = normalizeDataRoute(prefetchDataRoute);
-    } else if (isAppPPREnabled) {
+    } else if (experimentalPPR) {
       throw new Error('Invariant: expected to find prefetch data route PPR');
     }