AbstractException.php: fixed access to non existing array element.

extension/qfq/qfq/exceptions/AbstractException.php

@@ -135,7 +135,7 @@ class AbstractException extends \Exception {
 
                 $arrMerged = OnArray::htmlentitiesOnArray(array_merge($arrMsg, $arrDebugShow));
 
-                if (!empty($os = $arrMerged[ERROR_MESSAGE_OS])) {
+                if (!empty($os = $arrMerged[ERROR_MESSAGE_OS] ?? '')) {
                     $beforeMatch = htmlentities("the right syntax to use near '", ENT_QUOTES);
                     $afterMatch = htmlentities("' at line [0-9]*$", ENT_QUOTES);
                     if (preg_match("/mysqli: 1064.*$beforeMatch.*$afterMatch/", $os)) {