I just tested this out, and you can achieve having no text by using a zero-width space. Just copy the invisible character between the parenthesis here: () into the title field, or do the same from wikipedia, and then delete just the prenhesis. The icons will display as if there's no text, and no added padding to the right with a normal space.
Although, it may be nicer to have a toggle setting (perhaps for the entire bookmark bar and also on a per bookmark checkbox) that hides the tittle, so the bookmarks can still have a title, just not show it.