Major gateways say more ocean containers are shipping out empty, a sign of weak demand in troubled global markets and the tough sell American exporters face abroad ___ One of the fastest-growing U.S. exports right now is air. Shipments of empty containers out of the U.S. are surging this year, highlighting the impact the economic slowdown in China is having on U.S. exporters. The U.S. imports more from China than it sends back, but certain American industries—including those that supply scrap metal and wastepaper—feed China’s industrial production. Those exporters have suffered this year as China’s economy has cooled. In September,...