"No result" is what investigative roles receive for their action if they are roleblocked or similar. While in the majority of cases players are not informed that they have been blocked, if the player is expecting a result, it is terrible form to simply not tell the player anything (they may assume the mod missed their action or forgot to give their result). Therefore it is necessary to tell the player they obtained no results from their investigation.
"No result" extends to situations other than blocks in which a player believes that they have used an investigative role, but the role in fact failed to function. For example, a player who has been unknowingly vanillaised will get "no result" on any attempt to use a removed investigative role, because they don't actually have the role.
It should generally not be possible to obtain a "no result" when not blocked; if a role learns that nothing happened, that's not a "no result", it's a result of "nothing happened". For example, a Tracker should be told that his target did not visit anyone, and a Watcher should be told that his target was not visited by anyone. Giving "no result" in these situations is ambiguous and not normal.
It is important that moderators word "no result" PMs to be clearly distinct from "negative result" PMs. In Normal games, a common example of approved wording for a "no result" PM is "Your action failed, and you did not receive a result."