MDL-35620 question engine: should distinguish NULL and 0.

This only matters in an obscure edge case, but it is an edge case we hit
with one of the OU question types.

This load data code is processing the results of a LEFT JOIN, so is_null
is the correct logic.

question/engine/questionattemptstep.php

@@ -386,7 +386,7 @@ class question_attempt_step {
         $record = $currentrec;
         $data = array();
         while ($currentrec && $currentrec->attemptstepid == $attemptstepid) {
-            if ($currentrec->name) {
+            if (!is_null($currentrec->name)) {
                 $data[$currentrec->name] = $currentrec->value;
             }
             $records->next();