only check for a question of noqanda regex match if output if non-empty

... and add debug log message

I ran into issues without this because it took a while before an interactive started spewing output (with OpenFOAM-Extend).

@stdweird: please review
it's not about being slow... if there's no output for a while at the start, `run_cmd_qa` will exit after `n` loop iterations...

it's the command being executed which is slow (before producing output), not `run_cmd_qa`

I agree that the infinite loop thing on no output isn't the best thing ever, I'll try and figure out a way around it...