Lesson/MDL-13311: make it possible to have a short answer question using 0 as answer. (backport)

mod/lesson/action/continue.php

@@ -84,7 +84,9 @@
             $studentanswer = s(stripslashes_safe($useranswer));
             break;
          case LESSON_SHORTANSWER :
-            if (!$useranswer = $_POST['answer']) {
+            if (isset($_POST['answer'])) {
+                $useranswer = $_POST['answer'];
+            } else {
                 $noanswer = true;
                 break;
             }            

mod/lesson/action/updatepage.php

@@ -83,7 +83,7 @@
         for ($i = 0; $i < $lesson->maxanswers; $i++) {
             // strip tags because the editor gives <p><br />...
             // also save any answers where the editor is (going to be) used
-            if ((!empty($form->answer[$i]) and trim(strip_tags($form->answer[$i]))) or isset($form->answereditor[$i]) or isset($form->responseeditor[$i])) {
+            if ((isset($form->answer[$i]) and (trim(strip_tags($form->answer[$i]))) != '') or isset($form->answereditor[$i]) or isset($form->responseeditor[$i])) {
                 if ($form->answerid[$i]) {
                     $oldanswer = new stdClass;
                     $oldanswer->id = clean_param($form->answerid[$i], PARAM_INT);