CI: handle missing variables gracefully in [docker_registry_auth.sh]

Context

These variables, introduced in !8670 (merged), are set when building the images in the jobs, but they are not in the docker:merge_manifests job. This leads to errors on the master pipeline as seen here:

https://gitlab.com/tezos/tezos/-/jobs/4346986445#L49

They do not actually impact docker registry authentication, they are only used here for debug output. Therefore, their absence is not an error.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
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