Prevent adding an empty email option in the feedback app in guest mode

BUG=699287
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2742103002
Cr-Commit-Position: refs/heads/master@{#456223}

chrome/browser/resources/feedback/js/feedback.js

@@ -344,6 +344,9 @@ function initialize() {
       });
 
       chrome.feedbackPrivate.getUserEmail(function(email) {
+        // Never add an empty option.
+        if (!email)
+          return;
         var optionElement = document.createElement('option');
         optionElement.value = email;
         optionElement.text = email;