Function encode_optional_data 

pub fn encode_optional_data(data: Option<Bytes>) -> (Vec<u8>, bool)

Encodes optional data for protobuf messages.

Handles the data_is_zero flag convention used in starpc:

• None -> empty data, data_is_zero = false
• Some(empty) -> empty data, data_is_zero = true
• Some(data) -> data bytes, data_is_zero = false

Returns

A tuple of (data bytes, data_is_zero flag).