fix: wrong result for CSV EMPTY_FIELD_AS = STRING.

[youngsofun]

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Summary

when EMPTY_FIELD_AS = STRING.
Empty should be loaded as an empty string for both string not null and string null.
before this fix, it is loaded as NULL for string null.

• Fixes #[Link the issue here]

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[hantmac]

LGTM！