By Alexander Cornwell DUBAI (Reuters) -Prince William, second in line to the British throne, on Thursday visited the United Arab Emirates at a time when the former British protectorate has faced an unprecedented though mostly foiled string of missile and drone attacks. Kensington Palace said it is the Duke of Cambridge’s first official visit to the UAE. Six of its emirates ceased being British protectorates 50 years ago and formed a federation. The seventh joined in 1972. The visit, at the request of Britain’s foreign office, comes as it seeks to deepen trade relations with wealthy Gulf Arab s…