fix: guard nullable on setGuard

Author: PellegrinoDurante

src/Http/Resources/ApiCollection.php

@@ -17,7 +17,7 @@ class ApiCollection extends ResourceCollection
      */
     protected ?string $guard = null;
 
-    public function setGuard(string $guard): static
+    public function setGuard(?string $guard): static
     {
         $this->guard = $guard;
 

src/Http/Resources/Contracts/AllowableFields.php

@@ -149,7 +149,7 @@ protected function mapFields($request): array
         return $this->filterAllowedFields($fields);
     }
 
-    public function setGuard(string $guard): static
+    public function setGuard(?string $guard): static
     {
         $this->guard = $guard;