Removing empty files coming from missing packages

[laeti-tia]

When a file to be downloaded in the list of dependencies https://github.com/perfsonar/minor-packages/blob/5.1.0/opensearch/archive_ext_packages.yml is missing, an empty archive file is created by curl. Then the Debian repository management tool reprepro tries to include an empty file and stops on an error, failing to complete.

For improved reliability, the download script should remove the empty file created, still printing a warning, so that the other new packages can be included.