Support clouds without a config.jelly

[Vlatombe]

Follows up #7658

For consistency with 45e656c

Testing done

Tested interactively with CloudBees CI feature that depended on it.

Proposed changelog entries

• Fix support of clouds without a config.jelly file.

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
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Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

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[jglick]

Context: #7658 (comment)

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!