The dialog to select an article to translate (T111094) shows an red border around the input field when it becomes empty:
An empty search is a legitimate value that we should not be warning about, especially when we'll be showing some suggestions in that state.
More generally, in a search scenario I don't think we need to surface too much the idea that the user is doing something wrong. As long as the user is aware of the results that the search is producing, there is no need for alarming the user. More on empty results in a separate ticket (T177469)