fix: avoid null-pointer exception

src/Twig/AppRuntime.php

@@ -156,8 +156,12 @@ class AppRuntime implements RuntimeExtensionInterface
         return sprintf('<span class="%s">%s</span>', $class, $label);
     }
 
-    public function nl2List(string $content, string $class = 'list-disc pl-4'): string
+    public function nl2List(?string $content, string $class = 'list-disc pl-4'): string
     {
+        if (null === $content) {
+            return '';
+        }
+
         $items = explode(PHP_EOL, $content);
 
         $list = '<ul class="'.$class.'">';