drainer: process empty string from tidb in update/delete statement on oracle

[cartersz]

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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[cartersz]

/run-all-tests

[cartersz]

@lichunzhu PTAL

[lichunzhu]

/run-all-tests

[lichunzhu]

LGTM

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: dd7eb1c

[lichunzhu]

/hold

[cartersz]

/run-all-tests

[cartersz]

/run-all-tests

[lichunzhu]

/unhold

[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: 17b2b1b

[lichunzhu]

/cherry-pick release-6.1

[ti-chi-bot]

@lichunzhu: new pull request created: #1166.

In response to this:

/cherry-pick release-6.1

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