Fix handling of property drawers containing only whitespace.

src/parser/property_drawer.rs

@@ -78,7 +78,11 @@ pub(crate) fn property_drawer<'b, 'g, 'r, 's>(
         PropertyDrawer {
             source: source.into(),
             children,
-            contents: Some(contents.into()),
+            contents: if contents.len() > 0 {
+                Some(contents.into())
+            } else {
+                None
+            },
             post_blank: post_blank.map(Into::<&str>::into),
         },
     ))