[fix] Fix UnsupportedOperationException when update topic policies

[dao-jun]

Fixes #24043

Motivation

Fixes #24043

Modifications

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[BewareMyPower]

Could you replace it with an admin call? The original issue is reproduced by the bin\pulsar-admin topicPolicies set-subscription-dispatch-rate command to trigger the computeIfAbsent call:

**java.lang.UnsupportedOperationException**
        at java.base/java.util.Collections$EmptyMap.**computeIfAbsent**(Collections.java:4764)
        at org.apache.pulsar.broker.admin.impl.PersistentTopicsBase.lambda$**internalSetSubscriptionLevelDispatchRate**$487(PersistentTopicsBase.java:4960)

but the test code just test computeIfAbsent directly. If the code of internalSetSubscriptionLevelDispatchRate changed, this test would be meaningless.