fix: force attachment to use EmptyFieldAs::FieldDefault.

[youngsofun]

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Summary

CSV attachment is mainly used in Drivers for insert.
In the future, client should use EmptyFieldAs=STRING or FieldDefault to distinguish NULL and empty string.
However, old servers ds not support missing_field_as, so client can not add the option directly at now.
So we will get empty_field_as = NULL, which will raise error if there is empty string for non-nullable string field.

remove this after

1. the old server is no longer supported
2. Driver add the option "EmptyFieldAs=FieldDefault"

related pr #14335

• 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