fix: Fix import failed due to 0 row num

[bigsheeper]

issue: #39885

[tedxu]

/lgtm
/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bigsheeper, tedxu

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• internal/datanode/OWNERS [tedxu]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[mergify]

@bigsheeper go-sdk check failed, comment rerun go-sdk can trigger the job again.

[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[mergify]

@bigsheeper go-sdk check failed, comment rerun go-sdk can trigger the job again.

[codecov]

Codecov Report

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

Project coverage is 80.04%. Comparing base (a54c3fe) to head (a21078b).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/datanode/importv2/task_import.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #39886       +/-   ##
===========================================
+ Coverage   69.39%   80.04%   +10.65%     
===========================================
  Files         302     1488     +1186     
  Lines       27077   205833   +178756     
===========================================
+ Hits        18790   164763   +145973     
- Misses       8287    35113    +26826     
- Partials        0     5957     +5957     

Components	Coverage Δ
Client	79.24% <ø> (∅)
Core	69.40% <ø> (+<0.01%)	⬆️
Go	81.76% <50.00%> (∅)

Files with missing lines	Coverage Δ
internal/datanode/importv2/util.go	63.87% <100.00%> (ø)
internal/datanode/importv2/task_import.go	76.13% <40.00%> (ø)

... and 1185 files with indirect coverage changes

[tedxu]

rerun go-sdk

[tedxu]

/lgtm