[Dubbo-3625]Add constant 'MIN_PATH_ARRAY_LENGTH'

[John-Smile]

Fixes #3625.

What is the purpose of the change

Remove magic number in org.apache.dubbo.configcenter.support.zookeeper.CacheListener#childEvent

Brief changelog

Add constant 'MIN_PATH_ARRAY_LENGTH'.

Verifying this change

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[John-Smile]

Conflicts with new commits.