[Backport 6.1] fix(_get_keyspaces_to_decrease_rf): Address a case where no keyspace RF value of DC

[mergify]

In case no keyspace replication-factor value is retrieved in a DC,
A warning is logged and the keyspace is ignored (skipped).

Fixes: #8694

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit e57d75b)

Parent PR: #8695

[mergify]

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	both modified:   sdcm/utils/replication_strategy_utils.py

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[yarongilor]

Addressed in #8547