show import: fix show import might read data from prev line

[D3Hunter]

What problem does this PR solve?

Issue Number: ref #42930

Problem Summary:

What is changed and how it works?

• check isNull before get it, else we might got data of prev row

Check List

Tests

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

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

[GMHDBJD]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD, lance6716

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-16 06:16:20.380161388 +0000 UTC m=+325576.794765465: ☑️ agreed by GMHDBJD.
• 2023-06-16 06:41:20.298437762 +0000 UTC m=+327076.713041836: ☑️ agreed by lance6716.