make nonce optional

src/PersistentData/Model/OAuthSession.php

@@ -19,7 +19,7 @@ class OAuthSession extends Model
 
     private array $scope = [];
 
-    private string $nonce = '';
+    private ?string $nonce = '';
 
     private ?string $codeChallenge = null;
 
@@ -52,7 +52,7 @@ class OAuthSession extends Model
         $this->setScopeArray(explode(' ', $scope));
     }
 
-    public function setNonce(string $nonce): void
+    public function setNonce(?string $nonce): void
     {
         $this->nonce = $nonce;
     }
@@ -125,7 +125,7 @@ class OAuthSession extends Model
         return $this->scope;
     }
 
-    public function getNonce(): string
+    public function getNonce(): ?string
     {
         return $this->nonce;
     }