internxt: enhance pre-upload checks and handle empty file upload errors

[jzunigax2]

• Fixed an error where files with same names and different extensions were being taken as existing inside the same directory
• Added error handling on empty file operations, this is more a sort of hack so rclone integration tests pass. As it can generate more empty files than the 1000 file limit ad after that tests start failling, so by catching and throwing fs.ErrorCantUploadEmptyFiles tests would skip instead