refactor: allow nullable metadata in MessageMetadataDialog

The metadata prop was previously required, but callers sometimes pass
null or undefined. Updating the type to allow null/undefined prevents
runtime errors and simplifies usage.

web-app/src/containers/dialogs/MessageMetadataDialog.tsx

@@ -58,7 +58,7 @@ interface MessageMetadata {
 }
 
 interface MessageMetadataDialogProps {
-  metadata: MessageMetadata // Use the specific interface
+  metadata: MessageMetadata | null | undefined // Use the specific interface
   triggerElement?: React.ReactNode
 }