In Catan, if you roll a seven and move the robber, do you actually have to rob someone?
From the game rules

You must move the robber immediately to the number token of any other terrain hex or to the desert hex.

The keyword here being any. So yes, you can move it to tiles without anyone on it.
Although I've never done this myself, I suppose there could be an argument made to do doing this for a couple reasons. One of them, like you mentioned, could be to appease other players. Another could be to try and allow someone to even the playing field. If player A is very ahead and has longest road, but player B is tied for longest road, you could place the robber on a unsettled tile as to not steal from Player B and allow them to try and take longest road which might even out the score a little bit.
However, an important distinction here is that this does not mean you cannot steal if you place it on a tile that does have settlements or cities on it. If you place the robber on a tile with other players cities or settlement, you must steal from one of them.