fix(pd): pd raft-follower failed to get leader address due to npe

[JisoLya]

Purpose of the PR

• close #xxx

Main Changes

Verifying these changes

• [ ] Trivial rework / code cleanup without any test coverage. (No Need)
• [ ] Already covered by existing tests, such as (please modify tests here).
• [ ] Need tests and can be verified as follows:
  • xxx

Does this PR potentially affect the following parts?

• [ ] Dependencies (add/update license info & regenerate_known_dependencies.sh)
• [ ] Modify configurations
• [ ] The public API
• [ ] Other affects (typed here)
• [ ] Nope

Documentation Status

• [ ] Doc - TODO
• [ ] Doc - Done
• [ ] Doc - No Need