express js & PostgreSQL Put request inserts null in undefined fields

I make an edit profile request where is possible to edit one or more fields. And when i make put request and want change only one field, every undefined field becomes null. How to pass undefined field? Every not mentioned field in request saves its value

//update profile 
app.put("/id/:id", async (req,res) => {
        const {id} = req.params;
        const {username,  email, userdescription, photo_url} = req.body;
        const ifEmailExists = await isEmailAvailable(email)
        const ifUsernameExists = await isUsernameAvailable(username)
        if(ifEmailExists && ifUsernameExists){
        const updUser = await pool.query(
            "UPDATE users SET (username,  email, userdescription, photo_url) = ($1, $2, $3, $4) WHERE id = $5",
            [username, email, userdescription, photo_url, id]);
        res.json(`User data was updated successfully !`);
        } else { if(!ifUsernameExists&& !ifEmailExists){
            res.json({"msg": "Username and email are already used !"});}
            else {
                    res.json({"msg": "Email is already used !"});
                    else { if(!ifUsernameExists){
                        res.json({"msg": "Username is already used !"});}
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