message: prevent adding empty entries in the append file
diff: @@ -74,6 +74,8 @@ impl Writer {
  /// The slice **must** contain all the bytes that needs to be written in the
  /// append only file.
  pub fn append_bytes(&mut self, bytes: &[u8]) -> Result<()> {
  + if bytes.is_empty() { return Ok(()) }
  +
  let len = bytes.len() as Size;
  let mut sz_buf = [0u8;SIZE_SIZE];
  write_size(&mut sz_buf, len);
  