api: support delete store label by setting it to empty

[disksing]

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

cannot delete store label.

What is changed and how it works?

delete label by setting it to empty.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Code changes

• Has HTTP API interfaces change

[lhy1024]

LGTM. do we need to add doc to mention it

[disksing]

LGTM. do we need to add doc to mention it

Good idea.

[codecov-io]

Codecov Report

Merging #2044 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2044      +/-   ##
==========================================
- Coverage   76.97%   76.74%   -0.24%     
==========================================
  Files         185      185              
  Lines       18829    18835       +6     
==========================================
- Hits        14493    14454      -39     
- Misses       3236     3272      +36     
- Partials     1100     1109       +9

Impacted Files	Coverage Δ
server/core/store.go	78.62% <100%> (+0.29%)	⬆️
server/config/util.go	75% <100%> (+3.57%)	⬆️
server/member/leader.go	73.97% <0%> (-7.15%)	⬇️
server/region_syncer/client.go	77.1% <0%> (-4.82%)	⬇️
server/schedulers/random_merge.go	60.71% <0%> (-3.58%)	⬇️
server/schedule/region_scatterer.go	77.65% <0%> (-3.2%)	⬇️
client/client.go	72.23% <0%> (-1.82%)	⬇️
server/grpc_service.go	57.23% <0%> (-1.73%)	⬇️
pkg/btree/btree.go	86.84% <0%> (-0.81%)	⬇️
server/handler.go	48.91% <0%> (-0.44%)	⬇️
... and 2 more

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[disksing]

/merge

[sre-bot]

/run-all-tests