message: fix Shadows::with_signal
diff: @@ -67,6 +67,7 @@ impl<'a> Shadows<'a> {
  .map(|shadow| shadow.into_cow_str())
  .collect::<Cow<_>>()
  .join(", ")
  + .apply(|shadow_style| not(shadow_style.is_empty()).then(|| shadow_style))
  });
  let mut this = Self::default();
  this.dynamic_css_props
  