QVariantMap variantMap;
    variantMap.insert("A", "1");
    variantMap.insert("B", QVariant());
    qDebug().noquote() << QJsonDocument::fromVariant(variantMap).toJson(QJsonDocument::Compact);

    // Qt 5.14.2 output: {"A":"1","B":null}
    // Qt 5.15.0 output: {"A":"1"}