Here in South Africa, one of the continent’s three largest economies, there is no United States ambassador 18 months after the last one departed, and no sign of one being nominated. South Africans have taken note. Their country, scarcely mentioned by the president, sits at the bottom of a continent Trump disparages.That’s a pity. South African goings-on are instructive. The country has just emerged, its democracy shaken but intact, from nine years of plundering, corrupt rule by former president Jacob Zuma.