anchovYu opened a new pull request, #36110:
URL: https://github.com/apache/spark/pull/36110
### What changes were proposed in this pull request?
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Add a legacy flag
`spark.sql.legacy.nullValueWrittenAsUnquotedEmptyStringCsv.enabled` for the
breaking changed introduced in https://github.com/apache/spark/pull/34853 and
https://github.com/apache/spark/pull/34905 (followup).
The flag is enabled by default, so the null values written as csv will
output an unquoted empty string. When the flag is disabled, the null will
output quoted empty string.
### Why are the changes needed?
The original commit is a breaking change, and breaking changes should be
encouraged to add a flag to turn it off for smooth migration between versions.
### Does this PR introduce _any_ user-facing change?
With the default value of the conf, there is no user-facing difference.
If users turn this conf off, they can restore the pre-change behavior.
### How was this patch tested?
Through unit tests.
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