scheduler: add more nil-check code in adjacent_region

[nanne007]

What problem does this PR solve?

Fix potential nil panic in adjust-region scheduler after pd leader changed. related pr is #1250

What is changed and how it works?

Just two nil check.

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[siddontang]

Thanks @lerencao

can you add a test to simulate nil store here?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4bb9b96). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master    #1403   +/-   ##
=========================================
  Coverage          ?   75.61%           
=========================================
  Files             ?      135           
  Lines             ?    13143           
  Branches          ?        0           
=========================================
  Hits              ?     9938           
  Misses            ?     2306           
  Partials          ?      899

Impacted Files	Coverage Δ
server/schedulers/adjacent_region.go	74.56% <50%> (ø)

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