var grTask = new GlideRecord("task");
    grTask.addEncodedQuery("cmdb_ci.u_asset_s_vendor!=");
    grTask.query();
    while (grTask.next()) {
        var assetVendor = grTask.cmdb_ci.u_asset_s_vendor;
        grTask.u_case_s_vendor = assetVendor;
        grTask.update();

    }

Please mark Correct and Helpful if my answer helps you resolve your issue. Thanks!
Martin Ivanov
Community Rising Star 2022