🐛(frontend) revert || to ?? operator for metadata parsing

Fix broken parsing when metadata contains empty strings. Nullish coalescing
operator (??) doesn't handle empty strings like logical OR (||) does.

src/frontend/src/features/rooms/livekit/hooks/useRaisedHand.ts

@@ -9,7 +9,7 @@ type useRaisedHandProps = {
 export function useRaisedHand({ participant }: useRaisedHandProps) {
   // fixme - refactor this part to rely on attributes
   const { metadata } = useParticipantInfo({ participant })
-  const parsedMetadata = JSON.parse(metadata ?? '{}')
+  const parsedMetadata = JSON.parse(metadata || '{}')
 
   const toggleRaisedHand = () => {
     if (isLocal(participant)) {