distribution: registry: do not access the errors slice if it's empty

Signed-off-by: Antonio Murdaca <runcom@redhat.com>
(cherry picked from commit 0186f4d4223a094a050d06f456355da3ae431468)

CHANGELOG.md

@@ -16,7 +16,8 @@ be found.
 
 ### Distribution
 
-- Fix a crash when pushing multiple images sharing the same layers to the same repository in parallel  [#20831](https://github.com/docker/docker/pull/20831)
+- Fix a crash when pushing multiple images sharing the same layers to the same repository in parallel [#20831](https://github.com/docker/docker/pull/20831)
+- Fix a panic when pushing images to a registry which uses a misconfigured token service [#21030](https://github.com/docker/docker/pull/21030)
 
 ### Plugin system