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
this is not acceptable, as it allows to be stuck here forever (eg a hanging process with no output).
i fail to understand what speed is gained here. the onlything slow here is the sleep at the end of the loop