WIP Harden the replicas check

[apolukhin]

Harden the checks

Pull Request check-list

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• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
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• Is there an example added to the examples folder (if applicable)?
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Description of change

On MacOS the get_replicas() call returns empty list, even though the cluster_nodes() returns primaries and replicas and RedisCluster was created after the replicas negotiated with masters. This PR attempts to harden the checks, to make sure that replicas exist.