Added some notes about observed behavior and fixed return types for missing/absent keys.

src/renderer/renderer.rs

@@ -733,7 +733,12 @@ impl<'a> DustRenderer<'a> {
                         .map(|ce| ce.get_size())
                 });
                 match value_ce {
-                    None | Some(Err(_)) | Some(Ok(None)) => return Ok("".to_owned()),
+                    // If we found the key but could not get the size from it, render nothing.
+                    Some(Ok(None)) => return Ok("".to_owned()),
+                    // If "key" is not on the @size tag at all, render 0.
+                    None => return Ok("0".to_owned()),
+                    // If the key value could not be found in the context, render 0.
+                    Some(Err(_)) => return Ok("0".to_owned()),
                     Some(Ok(Some(ce_size))) => {
                         return ce_size.get_context_element_reference().render(&Vec::new())
                     }