EXAMPLE: Firebase with SignInWithApple on Unity

[bhupiister]

I came around same issue. Authorization code was null when using QuickLogin after using LoginWithAppleId.
I was using it to authenticate firebase and luckily Firebase is ok with AuthorizationCode being null.
Firebase.Auth.Credential credential = Firebase.Auth.OAuthProvider.GetCredential("apple.com", appleIdToken, rawNonce, null); Firebase GetCredential Documentation
AuthorizationCode will only be passed once, as @lupidan said.
@lupidan I would suggest you to modify your firebase example with unity and make 4th parameter of GetCredential as null.

I'll leave a code for anyone wanting help on linking firebase with Apple ID

Spoiler: I am making a guest account first and then linking it with the Apple ID(Firebase) because of my game logic. You can use
SignInWithCredentialAsync instead of LinkAndRetrieveDataWithCredentialAsync

public void SignInWithAppleID()//Sing in with Apple Button command here
{
    SignInWithAppleID(userReceived =>
    {
        if (userReceived != null)
        {
            user = userReceived;//If everything is successful, this will be current logged in firebase user.
            Debug.Log(user.UserId);//This will give firebase userid
        }
        else
        {
            //Failed to receive firebase user
        }
    });
}
public void SignInWithAppleID(Action<FirebaseUser> firebaseAuthCallback)
{

    string rawNonce = GenerateRandomString(32);//See lupidan example for this function
    string nonce = GenerateSHA256NonceFromRawNonce(rawNonce);//See lupidan example for this function
    AppleAuthQuickLoginArgs quickLoginArgs = new AppleAuthQuickLoginArgs(nonce);

    // Quick login should succeed if the credential was authorized before and not revoked
    appleAuthManager.QuickLogin(quickLoginArgs, credential =>
    {
        var appleIdCredential = credential as IAppleIDCredential;
        if (appleIdCredential != null)
        {
            // If it's an Apple credential, save the user ID, for later logins
            PlayerPrefs.SetString(AppleUserIdKey, credential.User);            
            PerformFirebaseAuthentication(appleIdCredential, rawNonce, firebaseAuthCallback);
        }
    },
    error =>
    {
        // If Quick Login fails, we should show the normal sign in with apple menu, to allow for a normal Sign In with apple
        var authorizationErrorCode = error.GetAuthorizationErrorCode();
        Debug.LogWarning("Quick Login Failed " + authorizationErrorCode.ToString() + " " + error.ToString());
        SignInWithApple(userReceived =>
        {
            user = userReceived;
            if (user != null)
            {
                firebaseAuthCallback(user);
            }
            else
            {
                //Firebase auth was cancelled or failed
                firebaseAuthCallback(null);
            }
        });
    });
}

private void SignInWithApple(Action<FirebaseUser> firebaseAuthCallback)
{
    string rawNonce = GenerateRandomString(32);
    string nonce = GenerateSHA256NonceFromRawNonce(rawNonce);
    AppleAuthLoginArgs loginArgs = new AppleAuthLoginArgs(LoginOptions.IncludeEmail | LoginOptions.IncludeFullName, nonce);

    this.appleAuthManager.LoginWithAppleId(loginArgs, credential =>
    {
        // If a sign in with apple succeeds, we should have obtained the credential with the user id, name, and email, save it
        var appleIdCredential = credential as IAppleIDCredential;
        if (appleIdCredential != null)
        {
            PlayerPrefs.SetString(AppleUserIdKey, credential.User);
            PerformFirebaseAuthentication(appleIdCredential, rawNonce, firebaseAuthCallback);
        }
    },
    error =>
    {
        var authorizationErrorCode = error.GetAuthorizationErrorCode();
        Debug.LogWarning("Sign in with Apple failed " + authorizationErrorCode.ToString() + " " + error.ToString());
        firebaseAuthCallback(null);
    });
}

private void PerformFirebaseAuthentication(IAppleIDCredential appleIdCredential, string rawNonce, Action<FirebaseUser> firebaseAuthCallback)
{
    string identityToken = "";
    string authorizationCode = "";
    if (appleIdCredential.IdentityToken != null)
    {
        identityToken = Encoding.UTF8.GetString(appleIdCredential.IdentityToken, 0, appleIdCredential.IdentityToken.Length);
    }
    else
    {
        firebaseAuthCallback(null);//Return null if identityToken is null as we cannot proceed without it.
    }
    if (appleIdCredential.AuthorizationCode != null)//This will not be null for the first time, for all subsequent calls, it will return null.
    {
        authorizationCode = Encoding.UTF8.GetString(appleIdCredential.AuthorizationCode, 0, appleIdCredential.AuthorizationCode.Length);
    }
    else
    {
        //If Authorization code is null, proceed without it and make it the string null. Firebase does not require it.
        authorizationCode = null;
    }
    Credential credential = OAuthProvider.GetCredential("apple.com", identityToken, rawNonce, authorizationCode);//authorizationCode can be null
    auth.CurrentUser.LinkAndRetrieveDataWithCredentialAsync(credential).ContinueWithOnMainThread(task =>
    {
        if (task.IsCanceled)
        {
            //Task is cancelled
            firebaseAuthCallback(null);
        }
        else if (task.IsFaulted)
        {
            //Task is faulted
            Debug.LogError("linkAsync encountered an error: " + task.Exception);     
            firebaseAuthCallback(null);
           
        }
        else if (task.IsCompleted)
        {               
            firebaseAuthCallback(auth.CurrentUser);        
        }        
    });

}

[lupidan]

Yeah, I guess one side effect from the authorization code being null after the first time is that the example is no longer valid and should probably be updated.

However the identity token should never be null AFAIK, so only a null check on the authorization code should be required.
The documentation needs to be updated yes 👍