Support empty sections under headings.

src/parser/document.rs

@@ -290,6 +290,13 @@ fn _heading<'r, 's>(
     if let Some(section) = maybe_section {
         children.insert(0, section);
     }
+    let remaining = if children.is_empty() {
+        // Support empty headings
+        let (remain, _ws) = many0(blank_line)(remaining)?;
+        remain
+    } else {
+        remaining
+    };
 
     let source = get_consumed(input, remaining);
     Ok((