feat(ui): add valueOrNull to useMetadataItem

In order to allow for null and undefined metadata values, this hook returned a symbol to indicate that parsing failed or was pending.

For values where the parsed value will never be null or undefined, it is useful get the value or null (instead of a symbol).

invokeai/frontend/web/src/features/metadata/hooks/useMetadataItem.tsx

@@ -72,5 +72,12 @@ export const useMetadataItem = <T,>(metadata: unknown, handlers: MetadataHandler
     handlers.recall(value, true);
   }, [handlers, value]);
 
-  return { label, isDisabled, value, renderedValue, onRecall };
+  const valueOrNull = useMemo(() => {
+    if (value === MetadataParsePendingToken || value === MetadataParseFailedToken) {
+      return null;
+    }
+    return value;
+  }, [value]);
+
+  return { label, isDisabled, value, renderedValue, onRecall, valueOrNull };
 };