core: fix domain group delete (#994)

packages/hydrooj/src/handler/domain.ts

@@ -434,7 +434,7 @@ export const DomainApi = {
         }),
         async (ctx, args) => {
             if (!ctx.user.hasPerm(PERM.PERM_EDIT_DOMAIN)) throw new PermissionError(PERM.PERM_EDIT_DOMAIN);
-            if (args.uids) {
+            if (args.uids?.length) {
                 const res = await user.updateGroup(ctx.domain._id, args.name, args.uids);
                 return res.upsertedCount > 0;
             }