enhance: Bulkinsert supports null in csv formats

[OxalisCu]

see details in this issue #35911

[codecov]

Codecov Report

Attention: Patch coverage is 73.46939% with 13 lines in your changes missing coverage. Please review.

Project coverage is 72.61%. Comparing base (b2eb9fe) to head (387f93f).
Report is 59 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/util/importutilv2/csv/row_parser.go	69.69%	5 Missing and 5 partials ⚠️
internal/util/importutilv2/reader.go	50.00%	1 Missing and 1 partial ⚠️
internal/util/importutilv2/option.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #35912      +/-   ##
==========================================
- Coverage   81.60%   72.61%   -9.00%     
==========================================
  Files        1264     1264              
  Lines      150692   150789      +97     
==========================================
- Hits       122976   109488   -13488     
- Misses      22826    36406   +13580     
- Partials     4890     4895       +5     

Files with missing lines	Coverage Δ
internal/util/importutilv2/csv/reader.go	71.92% <100.00%> (ø)
internal/util/testutil/test_util.go	93.29% <100.00%> (+0.03%)	⬆️
internal/util/importutilv2/option.go	67.92% <83.33%> (+1.96%)	⬆️
internal/util/importutilv2/reader.go	65.38% <50.00%> (-4.19%)	⬇️
internal/util/importutilv2/csv/row_parser.go	69.66% <69.69%> (-0.10%)	⬇️

... and 239 files with indirect coverage changes

[smellthemoon]

seems no need to set a 'defaultNullKey', because reader only try to parse null when nullable. Can simply just return 'NULL' as the null key.

[OxalisCu]

This function is used to parse the nullkey configured by the user. In addition to the defaultNullKey, the user can configure other nullkeys to increase the flexibility of the export format.
For example, the NULL data exported by pg supports configuration http://www.postgres.cn/docs/13/sql-copy.html.
If it is not necessary, I can directly return "NULL".

[smellthemoon]

This function is used to parse the nullkey configured by the user. In addition to the defaultNullKey, the user can configure other nullkeys to increase the flexibility of the export format.
For example, the NULL data exported by pg supports configuration http://www.postgres.cn/docs/13/sql-copy.html.
If it is not necessary, I can directly return "NULL".

I see. it's ok for me now to follow the pg support configuration.

[smellthemoon]

/lgtm

[congqixia]

/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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