cluster: PDInstance PostRestart to check for nil updcfg

[sgykfjsm]

What problem does this PR solve?

close #2696

What is changed and how it works?

Add a nil check to prevent a dereference of updcfg during the standard cluster startup process.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[pingcap-cla-assistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.35%. Comparing base (5a78bf3) to head (201788f).

Files with missing lines	Patch %	Lines
pkg/cluster/spec/pd.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2697      +/-   ##
==========================================
- Coverage   42.36%   42.35%   -0.01%     
==========================================
  Files         424      424              
  Lines       47018    47018              
==========================================
- Hits        19917    19911       -6     
- Misses      24412    24418       +6     
  Partials     2689     2689              

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