schedule: fix panic when setting store-limit-mode to auto (#2544)

[ti-srebot]

cherry-pick #2544 to release-4.0

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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 #2547 into release-4.0 will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@               Coverage Diff               @@
##           release-4.0    #2547      +/-   ##
===============================================
- Coverage        77.16%   77.15%   -0.02%     
===============================================
  Files              204      204              
  Lines            21940    21944       +4     
===============================================
  Hits             16931    16931              
- Misses            3724     3727       +3     
- Partials          1285     1286       +1     

Impacted Files	Coverage Δ
pkg/mock/mockoption/mockoption.go	93.81% <33.33%> (-1.94%)	⬇️
server/schedule/operator_controller.go	82.21% <50.00%> (+1.37%)	⬆️
server/kv/etcd_kv.go	75.32% <0.00%> (-12.99%)	⬇️
server/schedulers/base_scheduler.go	60.86% <0.00%> (-8.70%)	⬇️
server/region_syncer/client.go	81.67% <0.00%> (-3.06%)	⬇️
server/cluster/coordinator.go	74.52% <0.00%> (-2.20%)	⬇️
server/schedulers/adjacent_region.go	75.53% <0.00%> (-1.07%)	⬇️
pkg/btree/btree.go	86.84% <0.00%> (-0.81%)	⬇️
server/handler.go	51.18% <0.00%> (-0.44%)	⬇️
server/cluster/cluster.go	80.58% <0.00%> (+0.43%)	⬆️
... and 8 more

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[lhy1024]

/merge

[ti-srebot]

/run-all-tests