Fix cache miss and delete an useless test

[shiyuhang0]

What problem does this PR solve?

part of #2521

1. getRegionStorePairByKey will return Pair<TiRegion, null> after we use the upstream client-java.
   this pr optimizes the logic to handle the store is null.
2. Delete batch get retry test because
   • this test is not right for the bug it fixed
   • the test will be done in the upstream client-java [close #684] Fix batch get blocked by write tikv/client-java#685

What is changed and how it works?

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
• Need to update the tidb-ansible repository
• Need to be included in the release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Daemonxiao
• zhangyangyu

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[shiyuhang0]

/run-all-tests

[shiyuhang0]

/run-all-tests

[shiyuhang0]

/run-all-tests

[shiyuhang0]

/run-all-tests test-flash=true

[shiyuhang0]

/run-all-tests test-flash=true

[shiyuhang0]

/run-all-tests

[shiyuhang0]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 20ea7d0