Display prompt in the notebook correctly

IPython/html/static/notebook/js/codecell.js

@@ -410,7 +410,7 b' define(['
 
     CodeCell.input_prompt_classical = function (prompt_value, lines_number) {
         var ns;
-        if (prompt_value === undefined) {
+        if (prompt_value === undefined || prompt_value === null) {
             ns = " ";
         } else {
             ns = encodeURIComponent(prompt_value);