FileNotFoundException on first authentication attempt

[Peaj]

File.ReadAllText in LoadPKCEToken fails on first authentication attempt because the file has not been created yet.
The code only checks if the file is empty but throws an exception if it does not exist.
A simple File.Exists check should fix it.

Tested in Unity Editor version 2021.1.12f2

FileNotFoundException: Could not find file "G:\Unity\Projects\Spotify4Unity\Spotify4Unity\PKCE-credentials.json"
System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) (at <695d1cc93cca45069c528c15c9fdd749>:0)
System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.IO.FileOptions options, System.String msgPath, System.Boolean bFromProxy, System.Boolean useLongPath, System.Boolean checkHost) (at <695d1cc93cca45069c528c15c9fdd749>:0)
(wrapper remoting-invoke-with-check) System.IO.FileStream..ctor(string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int,System.IO.FileOptions,string,bool,bool,bool)
System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize, System.Boolean checkHost) (at <695d1cc93cca45069c528c15c9fdd749>:0)
System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize) (at <695d1cc93cca45069c528c15c9fdd749>:0)
System.IO.StreamReader..ctor (System.String path, System.Boolean detectEncodingFromByteOrderMarks) (at <695d1cc93cca45069c528c15c9fdd749>:0)
System.IO.StreamReader..ctor (System.String path) (at <695d1cc93cca45069c528c15c9fdd749>:0)
(wrapper remoting-invoke-with-check) System.IO.StreamReader..ctor(string)
System.IO.File.ReadAllText (System.String path) (at <695d1cc93cca45069c528c15c9fdd749>:0)
PKCE_Authentification.LoadPKCEToken () (at Assets/Spotify4Unity/Core/Managed Authentification/PKCE/PKCE_Authentification.cs:253)
PKCE_Authentification.HasPreviousAuthentification () (at Assets/Spotify4Unity/Core/Managed Authentification/PKCE/PKCE_Authentification.cs:123)
PKCE_Authentification.StartAuthentification () (at Assets/Spotify4Unity/Core/Managed Authentification/PKCE/PKCE_Authentification.cs:65)
SpotifyService.AuthorizeUser () (at Assets/Spotify4Unity/Core/Services/SpotifyService.cs:159)
AccountStatusController.OnSignIn () (at Assets/Spotify4Unity/Examples/Scripts/Landing Scene/AccountStatusController.cs:45)
AccountStatusController.<Start>b__3_0 () (at Assets/Spotify4Unity/Examples/Scripts/Landing Scene/AccountStatusController.cs:25)
UnityEngine.Events.InvokableCall.Invoke () (at <adfa4b62400849189388df71c9e26e89>:0)
UnityEngine.Events.UnityEvent.Invoke () (at <adfa4b62400849189388df71c9e26e89>:0)
UnityEngine.UI.Button.Press () (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/UI/Core/Button.cs:70)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/UI/Core/Button.cs:114)
UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:57)
UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/ExecuteEvents.cs:272)
UnityEngine.EventSystems.EventSystem:Update() (at Library/PackageCache/com.unity.ugui@1.0.0/Runtime/EventSystem/EventSystem.cs:501)