fix: handle empty auth result as null

src/PIResponse.php

@@ -108,7 +108,7 @@ class PIResponse
         }
 
         // Check that the authentication status is one of the allowed ones
-        $r = $map['result']['authentication'] ?: null;
+        $r = (!empty($map['result']['authentication'])) ? $map['result']['authentication'] : null;
         if (in_array(
             $r,
             [AuthenticationStatus::CHALLENGE, AuthenticationStatus::ACCEPT, AuthenticationStatus::REJECT],