schedule: fix panic when setting store-limit-mode to auto

[rleungx]

What problem does this PR solve?

Closes #2540.

What is changed and how it works?

This PR checks if the store limits of adding peer and removing peer are existed before calling Mode.

Check List

Tests

• Unit test

Release note

• Fix the issue that It may panic when setting store-limit-mode to auto

[codecov-commenter]

Codecov Report

Merging #2544 into master will increase coverage by 0.08%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #2544      +/-   ##
==========================================
+ Coverage   77.11%   77.19%   +0.08%     
==========================================
  Files         204      204              
  Lines       22067    22071       +4     
==========================================
+ Hits        17016    17037      +21     
+ Misses       3771     3753      -18     
- Partials     1280     1281       +1     

Impacted Files	Coverage Δ
pkg/mock/mockoption/mockoption.go	93.81% <33.33%> (-1.94%)	⬇️
server/schedule/operator_controller.go	79.87% <50.00%> (+1.33%)	⬆️
server/schedulers/base_scheduler.go	60.86% <0.00%> (-8.70%)	⬇️
pkg/metricutil/metricutil.go	86.11% <0.00%> (-5.56%)	⬇️
server/tso/tso.go	80.29% <0.00%> (-3.65%)	⬇️
server/grpc_service.go	58.99% <0.00%> (-1.17%)	⬇️
server/schedulers/adjacent_region.go	75.53% <0.00%> (-1.07%)	⬇️
server/core/storage.go	74.38% <0.00%> (-0.83%)	⬇️
server/server.go	76.48% <0.00%> (-0.46%)	⬇️
server/cluster/cluster.go	79.93% <0.00%> (-0.43%)	⬇️
... and 8 more

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[disksing]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

cherry pick to release-3.1 failed

[ti-srebot]

cherry pick to release-4.0 in PR #2547