It looks like the node in question is only doing one lookup. You can download the source and print out whatever information you want. The exception is being caught, so I don't think gdb is going to help you here. I'd remove that.
If you can't find any empty frames, try doing a rostopic echo /tf or a rostopic echo /tf_static to see if any of the raw tf data shows empty frames. You could grep for an empty frame_id or child_frame_id field. Then you can backtrack to whatever node is publishing the transform in question.