In Exercise 9.11, you wrote a Web page that selected a sequence of random letters from a string that was entered by the user. Similar to the modification in Exercise 11.2, modify your randSeq.html page so that it checks to make sure that the user has correctly entered characters to choose from. If not, i.e., if the text box is empty when the button is clicked, an alert window should appear to warn the user. Otherwise, the random sequence should be generated and displayed as before?