MDL-51837 user: Check if user email is set on validation

Add a check if the user's email is set before performing validation on
the user's email address. This is to prevent the warning messages to
be shown in the case of a pending email change which causes the user's
email field to be not included in the Edit profile form.

user/editadvanced_form.php

@@ -259,7 +259,7 @@ class user_editadvanced_form extends moodleform {
             }
         }
 
-        if (!$user or $user->email !== $usernew->email) {
+        if (!$user or (isset($usernew->email) && $user->email !== $usernew->email)) {
             if (!validate_email($usernew->email)) {
                 $err['email'] = get_string('invalidemail');
             } else if (empty($CFG->allowaccountssameemail)