mcs: fix panic of store not found

[rleungx]

What problem does this PR solve?

Issue Number: ref #5839.

What is changed and how does it work?

Fix the following issue:

[2025/09/26 01:52:51.583 +00:00] [WARN] [grpc_service.go:209] ["the store of the bucket in region is not found "] [region-id=46]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x29d5e4c]

goroutine 454 [running]:
[github.com/tikv/pd/pkg/core.(*StoreInfo).GetAddress(...)](http://github.com/tikv/pd/pkg/core.(*StoreInfo).GetAddress(...))
	/workspace/source/pd/pkg/core/store.go:337
[github.com/tikv/pd/pkg/mcs/scheduling/server.(*Service).RegionBuckets](http://github.com/tikv/pd/pkg/mcs/scheduling/server.(*Service).RegionBuckets)(0x400098a0a8, {0x34591d0, 0x40007612b0})
	/workspace/source/pd/pkg/mcs/scheduling/server/grpc_service.go:212 +0x1ac
[github.com/pingcap/kvproto/pkg/schedulingpb._Scheduling_RegionBuckets_Handler](http://github.com/pingcap/kvproto/pkg/schedulingpb._Scheduling_RegionBuckets_Handler)({0x3010060?, 0x400098a0a8}, {0x3452c98, 0x4000f881e0})
	/root/go/pkg/mod/[github.com/pingcap/kvproto@v0.0.0-20250923064352-8eeada0a8a03/pkg/schedulingpb/schedulingpb.pb.go:1713](http://github.com/pingcap/kvproto@v0.0.0-20250923064352-8eeada0a8a03/pkg/schedulingpb/schedulingpb.pb.go:1713) +0xdc
[google.golang.org/grpc.(*Server).processStreamingRPC](http://google.golang.org/grpc.(*Server).processStreamingRPC)(0x40009d4a00, {0x344d668, 0x400097dce0}, {0x345ab20, 0x4000488b60}, 0x4000856a20, 0x40008ffa70, 0x477b120, 0x0)
	/root/go/pkg/mod/[google.golang.org/grpc@v1.62.1/server.go:1680](http://google.golang.org/grpc@v1.62.1/server.go:1680) +0xe60
[google.golang.org/grpc.(*Server).handleStream](http://google.golang.org/grpc.(*Server).handleStream)(0x40009d4a00, {0x345ab20, 0x4000488b60}, 0x4000856a20)
	/root/go/pkg/mod/[google.golang.org/grpc@v1.62.1/server.go:1801](http://google.golang.org/grpc@v1.62.1/server.go:1801) +0xadc
[google.golang.org/grpc.(*Server).serveStreams.func2.1()](http://google.golang.org/grpc.(*Server).serveStreams.func2.1())
	/root/go/pkg/mod/[google.golang.org/grpc@v1.62.1/server.go:1027](http://google.golang.org/grpc@v1.62.1/server.go:1027) +0x84
created by [google.golang.org/grpc.(*Server).serveStreams.func2](http://google.golang.org/grpc.(*Server).serveStreams.func2) in goroutine 444
	/root/go/pkg/mod/[google.golang.org/grpc@v1.62.1/server.go:1038](http://google.golang.org/grpc@v1.62.1/server.go:1038) +0x13c

Check List

Tests

• Unit test

Release note

None.

[bufferflies]

The address can be nil if the store is nil

[rleungx]

Yes, it won't get the address if it is nil.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.77%. Comparing base (0b32e6e) to head (35f58f3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9781      +/-   ##
==========================================
- Coverage   76.87%   76.77%   -0.10%     
==========================================
  Files         488      488              
  Lines       77722    77727       +5     
==========================================
- Hits        59751    59678      -73     
- Misses      14337    14400      +63     
- Partials     3634     3649      +15     

Flag	Coverage Δ
unittests	76.77% <85.71%> (-0.10%)	⬇️

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[rleungx]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bufferflies, lhy1024

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• OWNERS [bufferflies,lhy1024]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-26 02:59:42.525183163 +0000 UTC m=+584792.595676838: ☑️ agreed by lhy1024.
• 2025-09-26 03:10:16.322291444 +0000 UTC m=+585426.392785125: ☑️ agreed by bufferflies.

[ti-chi-bot]

@rleungx: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
non-block/pull-unit-test-next-gen	35f58f3	link	false	/test pull-unit-test-next-gen

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[rleungx]

/retest