server: fix placement rules panic

[disksing]

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

What problem does this PR solve?

Fix #2069

What is changed and how it works?

Check if bootstrapped.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  when not bootstrapped:

./bin/pd-ctl config set enable-placement-rules true
Failed to set config: [500] "TiKV cluster not bootstrapped, please start TiKV first"

[codecov-io]

Codecov Report

Merging #2070 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2070      +/-   ##
==========================================
- Coverage   76.61%   76.54%   -0.07%     
==========================================
  Files         190      190              
  Lines       19279    19279              
==========================================
- Hits        14771    14758      -13     
- Misses       3378     3387       +9     
- Partials     1130     1134       +4

Impacted Files	Coverage Δ
server/config/config.go	89.47% <ø> (ø)	⬆️
pkg/etcdutil/etcdutil.go	82.6% <0%> (-5.8%)	⬇️
server/tso/tso.go	77.37% <0%> (-2.92%)	⬇️
server/member/leader.go	75.51% <0%> (-1.03%)	⬇️
pkg/btree/btree.go	86.84% <0%> (-0.81%)	⬇️
server/cluster/cluster.go	81.16% <0%> (-0.46%)	⬇️
server/server.go	81.95% <0%> (-0.19%)	⬇️
server/core/storage.go	72.39% <0%> (ø)	⬆️
server/config/option.go	91.24% <0%> (+0.92%)	⬆️
server/kv/etcd_kv.go	75.32% <0%> (+2.59%)	⬆️
... and 1 more

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[lhy1024]

/merge

[sre-bot]

/run-all-tests