fix: return fragment instead of null

or the website will be broken for some unknown reason 🤔

Author: pan93412

app/login/_components/posthog-resetter.tsx

@@ -9,5 +9,5 @@ export default function PostHogResetter() {
     posthog.reset();
   }, []);
 
-  return null;
+  return <></>;
 }

providers/posthog-identifier.tsx

@@ -20,5 +20,5 @@ export default function PostHogIdentifier() {
     }
   }, [user]);
 
-  return null;
+  return <></>;
 }