Support a no-op for headline pre-blank.

src/wasm/headline.rs

@@ -37,9 +37,13 @@ pub struct WasmHeadline {
     pub(crate) deadline: Option<Box<WasmAstNode>>,
     pub(crate) closed: Option<Box<WasmAstNode>>,
     #[serde(rename = "pre-blank")]
-    pub(crate) pre_blank: usize,
+    pub(crate) pre_blank: Noop,
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(tag = "noop")]
+pub struct Noop {}
+
 to_wasm!(
     WasmHeadline,
     Heading<'s>,
@@ -129,7 +133,7 @@ to_wasm!(
                     })
                     .map_or(Ok(None), |r| r.map(Some))?
                     .map(|child| Box::new(child)),
-                pre_blank: 0, // TODO: Should this be a no-op?
+                pre_blank: Noop {},
             },
         ))
     }