Change compression fn to save empty metadata dir

This commit fixes an issue when storing checkpoints with no metadata.
Now the metadata folder is stored in the compressed checkpoint even
if there are no files inside. This avoids an error when trying to load
the checkpoint.

Author: giorgio-mariani

src/nn_core/serialization.py

@@ -95,6 +95,13 @@ def compress_checkpoint(src_dir: Path, dst_file: Path, delete_dir: bool = True):
     with zipfile.ZipFile(_normalize_path(dst_file), "w") as zip_file:
         for folder, subfolders, files in os.walk(src_dir):
             folder: Path = Path(folder)
+            for subfolder in subfolders:
+                zip_file.write(
+                    folder / subfolder,
+                    (folder / subfolder).relative_to(src_dir),
+                    compress_type=zipfile.ZIP_DEFLATED,
+                )
+
             for file in files:
                 zip_file.write(
                     folder / file,