Proposed changes to empty command handling in JDB

[Jakob Cornell]

Hi Alan,

Yes, this is just the empty command handling.  I am personally interested in Jline integration, but Chris is skeptical about the review prospects for that, and since he's been helping me through the process I've limited the present work.  Maybe now that the patch for empty command handling is done (for now) I'll look into prototyping Jline integration.  I believe that would need a separate JBS ticket, and a separate PR once ready, and we probably want to kick off a new thread here since the present subject lines don't mention Jline.

Jakob