`TestDisableSchedulingServiceFallback` is flaky

[rleungx]

Flaky Test

Which jobs are failing

    server_test.go:109: 
        	Error Trace:	/home/runner/work/pd/pd/tests/integrations/mcs/scheduling/server_test.go:109
        	Error:      	Should NOT be empty, but was 
        	Test:       	TestServerTestSuite/TestAllocIDAfterLeaderChange

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x42186f0]

goroutine 450604 [running]:
github.com/tikv/pd/tests/integrations/mcs/scheduling.(*serverTestSuite).TestDisableSchedulingServiceFallback.func1()
	/home/runner/work/pd/pd/tests/integrations/mcs/scheduling/server_test.go:262 +0x50
github.com/stretchr/testify/assert.Eventually.func1()
	/home/runner/go/pkg/mod/github.com/stretchr/[email protected]/assert/assertions.go:1902 +0x34
created by github.com/stretchr/testify/assert.Eventually in goroutine 450613
	/home/runner/go/pkg/mod/github.com/stretchr/[email protected]/assert/assertions.go:1902 +0x3d6

CI link

https://github.com/tikv/pd/actions/runs/12367690342/job/34516396487?pr=8887

Reason for failure (if possible)

Anything else

[bufferflies]

let me fix it