Fix input empty as default for nullable string field in CSV InputFormat

[KevinyhZou]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix problems as described in https://github.com/ClickHouse/ClickHouse/pull/58225

While deserialize csv input, if we set the config input_format_csv_empty_as_default = true, which would set default value to field if the input is empty, and this is also take effect for Nullable(String) type, in this case, if the input is a empty string, then the field will be set default value as NULL, as show below

Here it is not suitable，to set NULL when the input is empty for Nullable(String) type, and it is better to set a empty string for the field.

Documentation entry for user-facing changes

• [ ] Documentation is written (mandatory for new features)

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