fix: use correct return type

src/BusProNet/Model/ProfileResponse.php

@@ -99,7 +99,7 @@ class ProfileResponse
         return $this;
     }
 
-    public function getDateOfBirth(): \DateTimeImmutable
+    public function getDateOfBirth(): ?\DateTimeImmutable
     {
         return $this->dateOfBirth;
     }