Normalize message for unanswered custom problems.

When a custom problem is not answered the validator is expected to
return an empty string result. The score is then marked as empty.
The special empty string is now converted to null right away, rather
than being passed as a message further into the framework.

This makes custom exercises display "more parts to answer" when a
user just clicks "Check Answer" without doing anything.

test/answer-types.js

@@ -985,6 +985,29 @@
         start();
     });
 
+    asyncTest("custom", 9, function() {
+        setupSolutionArea();
+        var $problem = jQuery("#qunit-fixture .problem").append(
+            "<div class='solution' data-type='custom'>" +
+                "<div class='instruction'>" +
+                    "<input type='text' id='custom-input'>" +
+                "</div>" +
+                "<div class='guess'>$('#custom-input').val()</div>" +
+                "<div class='validator-function'>" +
+                    "return guess == 'empty' ? '' : guess == 5;" +
+                "</div>" +
+            "</div>"
+        );
+
+        var answerData = Khan.answerTypes.custom.setup($("#solutionarea"),
+                $problem.children(".solution"));
+
+        testAnswer(answerData, "5", "right", "right answer is right");
+        testAnswer(answerData, "empty", "empty", "empty answer is empty");
+        testAnswer(answerData, "1", "wrong", "wrong answer is wrong");
+
+        start();
+    });
 
     asyncTest("prime factorization", 24, function() {
         setupSolutionArea();

utils/answer-types.js

@@ -1877,7 +1877,7 @@ Khan.answerTypes = $.extend(Khan.answerTypes, {
                     return {
                         empty: pass === "",
                         correct: pass === true,
-                        message: typeof pass === "string" ? pass : null,
+                        message: typeof pass === "string" && pass !== "" ? pass : null,
                         guess: guess
                     };
                 }