Critical issue on empty --limit option

var_dump(explode(',', ''));
prints the following output:

array(1) {
  [0]=>
  string(0) ""
}
therefore limit is never empty during the $configuration->isBackupActive() check, which leads to phpbu currently skipping all backups if you don't explicitly set the limit-option.

checking if the limit option is empty before explode'ing will fix the issue.