Jack was working at a breakneck pace illustrating the stories, then adding dialogue to his work, so this may be an example where he simply didn't have time to make sure his captions were perfectly composed. I'm not criticizing the work -- it's brilliantly executed and wonderfully delineated -- I just find that blank space puzzling from a storytelling perspective unless Kirby felt it would call more attention to the main character, which is entirely possible. The most likely possibility in my opinion: it could be that there was a word balloon there, and it was whited-out and removed during the editing process in New York at the DC offices. I'd love to take a look at the original artwork and see if there is evidence of an omission.
I put two white circles in the area of blank space to show how something like another caption or a piece of machinery might have added balance to the overall image if placed inside the smaller white circle.