What happens when you top directly above a node is that the highest branches, directly below where you cut, become their own main stems which will turn into colas.
You did fine, it makes no difference how much stem is left sticking up, the stems below will grow the same. There is no "Hole", the stem of MJ plants is hollow. Don't worry about the hollow stem either, the plant will fix that all by itself.
These are my 5 week old bubblegum that I topped 3 days ago, you can see I cut above the 2nd set of true nodes on each to create 4 main colas and that each plant indeed is hollow in the stem, although I have had strains that were not hollow, so I almost wonder if that has something to do with how strong vs lanky the plant is??