refactor: if cache_path is empty and enable_read_cache is true, set the default cache dir

Signed-off-by: zyy17 <zyylsxm@gmail.com>

src/datanode/src/store.rs

@@ -131,10 +131,8 @@ async fn build_cache_layer(
         _ => unreachable!("Already checked above"),
     };
 
-    // Enable object cache by default
-    // Set the cache_path to be `${data_home}` by default
-    // if it's not present
-    if cache_path.is_none() {
+    // If `cache_path` is unset or an empty string, default to use `${data_home}` as the local read cache directory.
+    if cache_path.as_ref().is_none_or(|p| p.is_empty()) {
         let read_cache_path = data_home.to_string();
         tokio::fs::create_dir_all(Path::new(&read_cache_path))
             .await