Please do not create a Pull Request without creating an issue first.

What is the purpose of the change

Support for unifying the empty namespace and Public namespace in the Nacos 3.0 configuration center. When storing data in the database, it is uniformly stored under the Public namespace, and it supports the smooth upgrade of empty namespace data from lower versions to the Public namespace.

Brief changelog

XX

Verifying this change

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Follow this checklist to help us incorporate your contribution quickly and easily:

• [*] Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• [*] Format the pull request title like [ISSUE #123] Fix UnknownException when host config not exist. Each commit in the pull request should have a meaningful subject line and body.
• [*] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• [*] Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.
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