Fix crash when updating sha-pinned images with no "latest" tag

[deivid-rodriguez]

If there's no latest tag, we can't update sha pinned images since we don't know which SHA is the latest.

Previously Dependabot would show the error "Pull request already exists for library/alpine-3 with latest version latest" even if that wasn't the case.

updater | 2023/08/29 00:21:53 INFO <job_714994438> Checking if library/alpine-3 latest needs updating
updater | 2023/08/29 00:21:53 INFO <job_714994438> Latest version is latest
  proxy | 2023/08/29 00:21:53 [018] HEAD https://registry.opensource.zalan.do:443/v2/library/alpine-3/manifests/latest
  proxy | 2023/08/29 00:21:53 [018] 200 https://registry.opensource.zalan.do:443/v2/library/alpine-3/manifests/latest
updater | 2023/08/29 00:21:53 INFO <job_714994438> Pull request already exists for library/alpine-3 with latest version latest

Fixes (part of) #7387.

[janschill]

Ran into this issue as well. Anything stopping this from getting merged?

[abdulapopoola]

@deivid-rodriguez is this deployable?

[abdulapopoola]

@janschill this is now deployed

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ RestClient::ServerBrokeConnection: Server broke connection docker/lib/dependabot/docker/update_checker.rb ... View Issue

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[deivid-rodriguez]

Honestly I don't recall, but I believe while a harmless change, it did not fix the original issue reported in #7387, that's why I had not merged it.