fix: show missing fields' label in flash message

src/Twig/AppRuntime.php

@@ -226,7 +226,8 @@ class AppRuntime implements RuntimeExtensionInterface
                     $labels = array_merge($labels, $this->collectInvalidFieldLabels($child));
                 } else {
                     // Leaf field, expanded choice, or compound field with own errors
-                    $labels[] = $child->vars['label'] ?? $child->vars['name'];
+                    $label = $child->vars['label'] ?? null;
+                    $labels[] = (\is_string($label) && '' !== $label) ? $label : $child->vars['name'];
                 }
             }
         }