ensure doc directory is not empty

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/doc.rs

@@ -743,7 +743,8 @@ fn run(self, builder: &Builder<'_>) {
             // Sanity check on linked compiler crates
             for krate in &*self.crates {
                 let dir_name = krate.replace("-", "_");
-                assert!(out.join(&*dir_name).exists());
+                // Making sure the directory exists and is not empty.
+                assert!(out.join(&*dir_name).read_dir().unwrap().next().is_some());
             }
         }