Fix post-blank for headlines containing only whitespace.

src/types/document.rs

@@ -36,6 +36,7 @@ pub struct Heading<'s> {
     pub deadline: Option<Timestamp<'s>>,
     pub closed: Option<Timestamp<'s>>,
     pub contents: Option<&'s str>,
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug)]
@@ -105,7 +106,13 @@ impl<'s> StandardProperties<'s> for Heading<'s> {
         self.children
             .last()
             .map(|child| child.get_post_blank())
-            .unwrap_or(0)
+            .unwrap_or(
+                self.post_blank
+                    .map(|text| text.lines().count())
+                    .unwrap_or(0)
+                    .try_into()
+                    .expect("Too much post-blank to fit into a PostBlank."),
+            )
     }
 }