Fix handling of documents containing only whitespace.

src/types/document.rs

@@ -17,7 +17,7 @@ pub struct Document<'s> {
     pub path: Option<PathBuf>,
     pub zeroth_section: Option<Section<'s>>,
     pub children: Vec<Heading<'s>>,
-    pub contents: Option<&'s str>,
+    pub contents: &'s str,
 }
 
 #[derive(Debug)]
@@ -64,7 +64,7 @@ impl<'s> StandardProperties<'s> for Document<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        self.contents
+        Some(self.contents)
     }
 
     fn get_post_blank(&self) -> PostBlank {