fix: fix nil point error

[Rory-Z]

When the EMQX resource is in coreNodesProgressing/replicantNodesProgressing state, if the user manually deletes currentSts/currentRs, it will cause a nil point error.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (14e0ed9) 77.00% compared to head (3ce4988) 77.26%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1006 +/- ## ========================================== + Coverage 77.00% 77.26% +0.26% ========================================== Files 67 67 Lines 6153 6153 ========================================== + Hits 4738 4754 +16 + Misses 1190 1175 -15 + Partials 225 224 -1 ```

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