Modify xbcloud to upload backups with empty database to min.io storage

OS: Ubuntu 18.0

xtrabackup version: 8.0.9 based on MySQL server 8.0.18 Linux (x86_64) (revision id: c5cbbe4)

Command line:

xtrabackup --verbose --backup --compress --stream=xbstream --extra-lsndir=/tmp | xbcloud --defaults-file=xbcloud.cnf --insecure put --s3-bucket='backups' --parallel=10 testdb_mysql-8.0.17_full_backup_2020-03-16-124010

Error seen:

200316 12:53:22 xbcloud: Failed to upload object backups/testdb_mysql-8.0.17_full_backup_2020-03-16-125312/test_empty_db//db.opt.qp.00000000000000000000. Error message: Object name contains unsupported characters.
200316 12:53:22 xbcloud: error: failed to upload chunk: testdb_mysql-8.0.17_full_backup_2020-03-16-125312/test_empty_db//db.opt.qp.00000000000000000000, size: 102
200316 12:53:22 xbcloud: Failed to upload object backups/testdb_mysql-8.0.17_full_backup_2020-03-16-125312/test_empty_db//db.opt.qp.00000000000000000001. Error message: Object name contains unsupported characters.
200316 12:53:22 xbcloud: error: failed to upload chunk: testdb_mysql-8.0.17_full_backup_2020-03-16-125312/test_empty_db//db.opt.qp.00000000000000000001, size: 38
xtrabackup: Transaction log of lsn (20080922) to (20080952) was copied.
200316 12:53:23 completed OK!
200316 12:53:23 xbcloud: Upload failed.

This is seen when there is an empty MySQL database, adding a table will allow xtrabackup to complete correctly.