Don't crash when backing up the root directory because it has no name

yama/src/operations/storing.rs

@@ -323,7 +323,12 @@ pub fn store_without_pointer_ops<PT: ProgressTracker>(
     let chunk_ref = commands::store_tree_node(
         &pile,
         &RootTreeNode {
-            name: root_dir.file_name().unwrap().to_str().unwrap().to_owned(),
+            name: root_dir
+                .file_name()
+                .map(|s| s.to_str())
+                .flatten()
+                .unwrap_or("_root")
+                .to_owned(),
             node: root_node,
         },
     )?;