[Bug] ：When upgrading from Doris version 1.2.8 to version 2.0.0, during metadata compatibility testing,executing SHOW FRONTENDS resulted in an error

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

1.2.8

### What's Wrong?

When upgrading from Doris version 1.2.8 to version 2.0.0, during metadata compatibility testing, after connecting to the FE node, executing SHOW BACKENDSworked normally. However, executing SHOW FRONTENDS resulted in an error: [StmtExecutor.executeByLegacy():767] execute Exception. stmt[2, 422028d2ade449cd-b08684aa0f9b09e8] java.lang.NullPointerException: null.

### What You Expected?

If this error occurs, can I still upgrade from version 1.2.8 to version 2.0.0?

### How to Reproduce?

_No response_

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start by reproducing the 1.2.8-to-2.0.0 metadata compatibility upgrade and running SHOW FRONTENDS after connecting to the FE node; compare it with the working SHOW BACKENDS command. Inspect the reported StmtExecutor.executeByLegacy():767 exception and its full stack trace. Done means the upgrade outcome is established and SHOW FRONTENDS no longer fails, or the incompatibility is clearly documented.

Written by the indexing model from the issue text.