drainer: process empty string from tidb in update/delete statement on oracle (#1163)

[ti-chi-bot]

This is an automated cherry-pick of #1163

What problem does this PR solve?

Issue Number: ref #1162

What is changed and how it works?

Check value of column , if it is empty string , drainer generate "col is null" clause in sql statement.

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

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hi-rustin
• lichunzhu

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[lichunzhu]

/run-all-tests

[lichunzhu]

LGTM

[lichunzhu]

/run-integration-test

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: edd9d62

[lichunzhu]

/run-all-tests

[lichunzhu]

/run-integration-tests