The original Photos app that is included with the O beta has no issue with the icon. There was an update for Photos on the O beta which broke the icon. There has since been another update which fixed the icon.
O beta stock Photos app: Photos has icon
O beta updated Photos app: Photos has no icon
O beta updated (again) Photos app: Photos has icon
Basically, for that one update, one of Google's devs forgot to apply the icon to the app, which broke the icon when the app was updated. They've since fixed it.