Fix: check s3 image url existence

[Pradip-p]

Description

This PR improves the get_image_dir_url function by adding a check to verify the existence of the image directory in the S3 bucket. Previously, the function generated a URL without confirming whether the directory actually existed. With this change, the function now:

1. Uses minio_client.list_objects to check if any objects exist in the specified image_dir.
2. Returns the constructed URL if the directory exists.
3. Returns None if the directory does not exist.