New Yorkers may get plenty of Chock Full o'Nuts coffee, but of late they haven't been getting much Chock Full o'Nuts advertising. And those of you who remember the advertising are probably glad. The can of regular Chock Full o'Nuts coffee is the best-selling individual grocery product in New York, according to Richard N. Costello, agency president. But the sales here have been flat or slipping slightly, he said, and that is why the company is advertising again.