[WIP] config: fix panic when some field is empty

[lance6716]

What problem does this PR solve?

close https://github.com/pingcap/dm/issues/2028

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 dm/dm-ansible
• Need to be included in the release note

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