code: protected function validateToOne($value, ?string $field = null): bool
      {
          if (is_null($value)) {
              return true;
          }
  
          $dataPath = $field ? "/data/relationships/{$field}" : "/";
          $identifierPath = $field ? "/data/relationships/{$field}" : "/data";
  
          if (!$this->validateIdentifier($value, $dataPath)) {
              return false;
          }
  
          if (!$this->store->exists(new ResourceIdentifier($value))) {
              $this->resourceDoesNotExist($identifierPath);
              return false;
          }
  
          return true;
      }
docs: Validate a to-one relation.
  
  @param mixed $value
  @param string|null $field
  the relationship field name.
  @return bool