Fixed: null check for PHP 8.1 (#476)

Co-authored-by: Nicola Asuni <[email protected]>

tcpdf.php

@@ -7636,7 +7636,7 @@ public function Output($name='doc.pdf', $dest='I') {
 			$dest = $dest ? 'D' : 'F';
 		}
 		$dest = strtoupper($dest);
-		if ($dest[0] != 'F') {
+		if ($dest[0] != 'F' && $name !== null) {
 			$name = preg_replace('/[\s]+/', '_', $name);
 			$name = preg_replace('/[^a-zA-Z0-9_\.-]/', '', $name);
 		}