The Food and Drug Administration Wednesday approved a West German calorie-free sweetener for use in certain food products. Acesulfame potassium, which will be marketed in the United States as "Sunette," has been approved for use in chewing gum, dry beverage mixes, instant coffee, instant tea, gelatins, puddings and non-dairy creamers. Sunette is a product of the Hoechst Celanese Corp. of Somerville, N.J., a subsidiary of Hoechst AG of Frankfurt. The artificial sweetener is used in 20 other countries.