fix PHP8 warning due to wrong initialization format (#539)

Author: CPASimUSante

classes/question/numerical.php

@@ -83,7 +83,7 @@ protected function question_survey_display($response, $descendantsdata, $blankqu
         // Numeric.
         $questiontags = new \stdClass();
         $precision = $this->precise;
-        $a = '';
+        $a = new \StdClass();
         if (isset($response->answers[$this->id][0])) {
             $mynumber = $response->answers[$this->id][0]->value;
             if ($mynumber != '') {