message: Add error test case into signature_serializer.
diff: @@ -94,4 +94,14 @@ mod tests {
  assert_eq!(serialize_signature(msg).unwrap(), result)
  }
  +
  + #[test]
  + #[ignore] /* FIXME implement ignoring signature field as in python code */
  + fn signature_serialize_works_with_null() {
  + let data = r#"{"signature": null}"#;
  + let v: serde_json::Value = serde_json::from_str(data).unwrap();
  + let serialized = serialize_signature(v).unwrap();
  + println!("{:?}", serialized);
  + assert_eq!(serialized, "");
  + }
  }
  \ No newline at end of file
  