Report.php: Empty columns in a query throws an exception - fixed.

extension/qfq/qfq/report/Report.php

@@ -580,7 +580,8 @@ class Report {
         $flagControl = false;
         $flagOutput = true;
 
-        if ($columnName[0] === "_") {
+        // Empty columnsnames are allowed: check with isset
+        if (isset($columnName[0]) && $columnName[0] === "_") {
             $flagControl = true;
             $columnName = substr($columnName, 1);
         }