MDL-38397 question editing: only validate default mark if present.

Most qtypes have that field, but not all. multianswer is an example in core.
Also some contrib qtypes.

question/type/edit_question_form.php

@@ -656,7 +656,7 @@ public function validation($fromform, $files) {
         }
 
         // Default mark.
-        if ($fromform['defaultmark'] < 0) {
+        if (array_key_exists('defaultmark', $fromform) && $fromform['defaultmark'] < 0) {
             $errors['defaultmark'] = get_string('defaultmarkmustbepositive', 'question');
         }