Don't pass null to some string functions for PHP 8.1

Author: nickdekruijk

src/Controllers/ReportController.php

@@ -122,7 +122,7 @@ public function show($slug, $id)
             }
             $response .= '<tr>';
             foreach ($row as $column) {
-                $response .= '<td><div>' . htmlspecialchars($column) . '</div></td>';
+                $response .= '<td><div>' . htmlspecialchars($column ?: '') . '</div></td>';
             }
             $response .= '</tr>';
         }