fix(_get_keyspaces_to_decrease_rf): Address a case where no keyspace RF value of DC

[yarongilor]

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)

[yarongilor]

Running a small test in https://argus.scylladb.com/test/e6972842-591d-472f-9e39-f196d3670053/runs?additionalRuns[]=5e71277b-52d8-4fa9-ad60-8221d766f887

==> passed ok.
output example:

< t:2024-09-15 09:13:34,051 f:replication_strategy_utils.py l:185  c:sdcm.utils.replication_strategy_utils p:WARNING > Datacenter eu-west_nemesis_dc not found in replication strategy for keyspace keyspace1.
< t:2024-09-15 09:13:34,062 f:replication_strategy_utils.py l:185  c:sdcm.utils.replication_strategy_utils p:WARNING > Datacenter eu-west_nemesis_dc not found in replication strategy for keyspace mview.
< t:2024-09-15 09:13:34,062 f:replication_strategy_utils.py l:185  c:sdcm.utils.replication_strategy_utils p:WARNING > Datacenter eu-west_nemesis_dc not found in replication strategy for keyspace keyspace_new_dc.
< t:2024-09-15 09:13:36,256 f:db_log_reader.py l:125  c:sdcm.db_log_reader   p:DEBUG > 2024-09-15T09:13:36.196+00:00 add-remove-dc-master-db-node-5e71277b-1     !INFO | scylla[6600]:  [shard 0: gms] load_balancer - Examining DC eu-west_nemesis_dc (shuffle=false, balancing=true)
< t:2024-09-15 09:13:36,256 f:db_log_reader.py l:125  c:sdcm.db_log_reader   p:DEBUG > 2024-09-15T09:13:36.196+00:00 add-remove-dc-master-db-node-5e71277b-1     !INFO | scylla[6600]:  [shard 0: gms] load_balancer - Will drain node e5a06229-bdbe-4fa9-b8a2-d96a9bcc7a50 (being_decommissioned) from DC eu-west_nemesis_dc

[fruch]

@yarongilor please stop adding unneed spaces in commits/descriptions

[fruch]

LGTM