[profile::pkgrepo -> mirror-scripts] sync-recent-releases.sh doesn't properly detect empty files

[daniel-beck]

https://github.com/jenkins-infra/mirror-scripts/blob/8ee32b77054e22d062719749bbdd3d8ed6510c57/sync-recent-releases.sh#L21-L23

This check is always false, because an empty file will result in /.

>> Update artifacts on get.jenkins.io
/

Uploading /
2023-03-08 15:09:17.722 DEBUG - credential: account=prodjenkinsreleases endpoint=core.windows.net is_sas=False can_create_containers=True can_list_container_objects=True can_read_object=True can_write_object=True
2023-03-08 15:09:17.734 INFO - 
============================================
         Azure blobxfer parameters
============================================
         blobxfer version: 1.6.0
                 platform: Linux-4.15.0-206-generic-x86_64-with-glibc2.27
               components: CPython=3.8.0-64bit azstor.blob=1.4.0 azstor.file=1.4.0 crypt=2.4.2 req=2.21.0
       transfer direction: local -> Azure
                  workers: disk=16 xfer=32 md5=4 crypto=0
                 log file: None
                  dry run: False
              resume file: None
                  timeout: connect=10 read=200 max_retries=1000
                     mode: StorageModes.File
                  skip on: fs_match=False lmt_ge=False md5=True
        delete extraneous: False
                overwrite: False
                recursive: True
            rename single: False
              access tier: None
         chunk size bytes: 0
           one shot bytes: 0
         strip components: 0
         store properties: attr=False md5=True
           rsa public key: None
       local source paths: /srv/releases/jenkins/plugins
============================================
2023-03-08 15:09:17.734 INFO - blobxfer start time: 2023-03-08 15:09:17.734439+00:00
2023-03-08 15:09:17.741 DEBUG - initializing 4 MD5 processes
2023-03-08 15:09:17.747 DEBUG - spawning 16 disk threads
2023-03-08 15:09:17.770 DEBUG - spawning 32 transfer threads
2023-03-08 15:09:17.914 INFO - not overwriting remote file: mirrorbits/plugins/gitlab-branch-source/642.v9ed86b_b_54384/gitlab-branch-source.hpi (local: /srv/releases/jenkins/plugins/gitlab-branch-source/642.v9ed86b_b_54384/gitlab-branch-source.hpi)
(etc)