refactor: fix return type

Author: kenjis

system/Helpers/html_helper.php

@@ -179,7 +179,7 @@ function doctype(string $type = 'html5'): string
         $config   = new DocTypes();
         $doctypes = $config->list;
 
-        return $doctypes[$type] ?? false;
+        return $doctypes[$type] ?? '';
     }
 }
 

system/View/Parser.php

@@ -619,7 +619,7 @@ protected function applyFilters(string $replace, array $filters): string
             $replace = $this->config->filters[$filter]($replace, ...$param);
         }
 
-        return $replace;
+        return (string) $replace;
     }
 
     // Plugins