[roq-fix-bridge] ExecutionReport.order_id (37) will now always contain a non-empty string

[thraneh]

It was observed that some client libraries (quickfix) may require a non-empty value for order_id (37).

It is a required field for ExecutionReport. Previous implementation used the empty string.

New implementation will either use client_order_id (from OrderAck or OrderUpdate) or set this field to exec_id (17) if no client_order_id has been received yet.

In summary, rejects may use exec_id for order_id if a reject happens before a order request has been sent to the exchange.