Best retro I ever had, on a small team of seniors: we all sat down, looked at each other, agreed that a sprint happened and we couldn't think of anything that was good or bad about it. Then we called in our manager, who also acted as scrum master, so we could do planning for our next sprint. I thought this was reasonable enough - ostensibly we'd do retro and then planning back to back, and none of us minded the chance to take a few minutes and reflect if we had anything we should discuss.

By contrast, I've worked with scrum masters who were strict about the process and insisted _every_ retro needed to have at least one improvmenet or action item out of it, preferably more. I found this pointless and I've rarely seen them actually followed up on.