Fix handling post blank for org documents.

org_mode_samples/sections_and_headings/document_post_blank.org

@@ -0,0 +1,5 @@
+* foo
+
+
+
+

src/types/document.rs

@@ -68,10 +68,7 @@ impl<'s> StandardProperties<'s> for Document<'s> {
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        self.into_iter()
+        0
-            .last()
-            .map(|child| child.get_post_blank())
-            .unwrap_or(0)
     }
 }