[DISCUSS][Cherry-pick] #23757 makes replicateSubscriptionState nullable

[Zixuan Liu]

Hi all,

#23757 makes replicateSubscriptionState nullable, which is important for
supporting subscription replication on the namespace and topic levels.

So I want to cherry-pick #23757 to the branch-3.0, branch-3.3, branch-4.0.

Thanks,
Zixuan