fix region split: region split key should be a vaild key (#2101)

[ti-srebot]

cherry-pick #2101 to master
You can switch your code base to this Pull Request by using git-extras:

# In tispark repo:
git pr https://github.com/pingcap/tispark/pull/2102

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tispark.git pr/2102:master-3193bda438c6

---

What problem does this PR solve?

fix region split: region split key should be a vaild key

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