I run a late 2013 rMBP and a Windows desktop. AFAIK, Apple hasn't upgraded iTunes for Windows to 12 (no real reason to...). I think it's a bug in 12 because I get the same blank on the rMBP but the data is there on the Win desktop that's running iTunes 11. (In other words, the app update data is there, it's just not showing up in 12.)