'message_status' response optional parameters

[bugz...@gmail.com]

Hello.

As it is said http://api.textmagic.com/https-api/textmagic-api-commands#message_status
there are two optional parameters in 'message_status' command
response. They are: credits_cost and completed_time
And it is said that "They are shown only if message is in final
status".

I wonder what does it mean? Will this parameters set to null value, or
can they be totally absent in JSON response object?

Thank you.