The last task on the coding challenge asks you to analyze trade data to find out how often specific stocks are traded. Write a function count$\_$trade$\_$frequency$()$ that takes a trade data string and a stock symbol $($also a string$)$ as parameters. The function should return the number of times the stock symbol appears in the trade data. The capitalization of the symbol should not affect the count. Trades are separated by semicolons. If the empty string is passed as either argument the function should return $0.$