[fix][backend] Fix task npe.

[taoyifan89]

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[CozeLoop]

No risks identified; change appears safe.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...y/domain/task/service/taskexe/tracehub/backfill.go	75.00%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   68.50%   68.59%   +0.09%     
==========================================
  Files         585      585              
  Lines       54218    54192      -26     
==========================================
+ Hits        37140    37173      +33     
+ Misses      14238    14177      -61     
- Partials     2840     2842       +2     

Flag	Coverage Δ
unittests	68.59% <88.88%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...s/observability/application/convertor/task/task.go	80.97% <100.00%> (+6.67%)	⬆️
...d/modules/observability/domain/task/entity/task.go	47.00% <ø> (ø)
...omain/task/service/taskexe/tracehub/local_cache.go	81.25% <100.00%> (ø)
...main/task/service/taskexe/tracehub/span_trigger.go	76.96% <100.00%> (-0.24%)	⬇️
...domain/task/service/taskexe/tracehub/subscriber.go	70.27% <100.00%> (ø)
...s/observability/infra/mq/consumer/task_consumer.go	100.00% <100.00%> (+100.00%)	⬆️
...y/domain/task/service/taskexe/tracehub/backfill.go	65.94% <75.00%> (+0.48%)	⬆️

... and 2 files with indirect coverage changes

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