fix cluster.sUnsubscribe - make listener optional

[Clsan]

Description

• I don't see any good reason for cluster.client.sUnsubscribe function must hasve listener.
• So, I changed cluster.client.sUnsubscribe definition. (Make listener optional)

• When sUnsubscribe is called, client.unsubscribe is called. #
• It finally goes to pubSub.unsubscribe. #
• If we register listener on sUnsubscribe, pubSubClient's isActive is true due to pre-registered listener. #
• So, command is not actually issued. #
• And, also not disconnected. #

Describe your pull request here

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?