Assets: fix first hot reloading (bevyengine#9804)

# Objective

- Hot reloading doesn't work the first time it is used

## Solution

- Currently, Bevy processor:
  1. Create the `imported_assets` folder
  2. Setup a watcher on it
  3. Clear empty folders, so the `imported_assets` folder is deleted
4. Recreate the `imported_assets` folder and add all the imported assets
- On a first run without an existing `imported_assets` with some
content, hot reloading won't work as step 3 breaks the file watcher
- This PR stops the empty root folder from being deleted
- Also don't setup the processor internal asset server for file
watching, freeing up a thread

---------

Co-authored-by: Carter Anderson <[email protected]>

crates/bevy_asset/src/processor/mod.rs

@@ -506,7 +506,7 @@ impl AssetProcessor {
                                 .await?
                                 && contains_files;
                     }
-                    if !contains_files {
+                    if !contains_files && path.parent().is_some() {
                         if let Some(writer) = clean_empty_folders_writer {
                             // it is ok for this to fail as it is just a cleanup job.
                             let _ = writer.remove_empty_directory(&path).await;