importinto: fix null value for base64

[GMHDBJD]

What problem does this PR solve?

Issue Number: close #60099

Problem Summary:

What changed and how does it work?

• fix null value for base64 by check \N

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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I added contribute agreement accepted..

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Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/release-8.1-gsort-test@14ea5f3). Learn more about missing BASE report.

Additional details and impacted files

@@                         Coverage Diff                         @@
##             feature/release-8.1-gsort-test     #60102   +/-   ##
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  Coverage                                  ?   71.2196%           
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  Files                                     ?       1469           
  Lines                                     ?     424268           
  Branches                                  ?          0           
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  Hits                                      ?     302162           
  Misses                                    ?     101542           
  Partials                                  ?      20564           

Flag	Coverage Δ
unit	71.2196% <50.0000%> (?)

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Components	Coverage Δ
dumpling	52.9656% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.4521% <0.0000%> (?)

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[lance6716]

it's better to check isNull as the normal code path

isNull = !parser.cfg.NotNull &&
			slices.Contains(parser.cfg.Null, unescaped)

[GMHDBJD]

since fields are already decoded and generally not quoted, I don't think similar quoting‐related logic should be apply.

[lance6716]

it's not about quoted. User may want to insert a \N liternal value into table (so need to check cfg.NotNull), and may choose another value to represent NULL (so need to check cfg.Null)