write to billing log, even if there is not an onLogComplete callback set

[kchapple]

Short description of what this resolves:

The billing log was only being written to disk if the generator task set an onLogCompleteCallback. In the case of the X12Direct generator, it doesn't set a callback, so the log is never written. This change writes the process_bills.log to disk every time.

[stephenwaite]

bringing this in @kchapple @juggernautsei then will follow up with quick fix for bn_x12

thank you