fix: when bufferSize is zero, storage can not write data to file

[gaius-qi]

Description

• When bufferSize is zero, createDownload and createNetworkTopology can not write data to file, because buffer is empty.

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[fcgxz2003]

LGTM

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@39c9839). Click here to learn what that means.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2366   +/-   ##
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  Coverage        ?   49.69%           
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  Files           ?      148           
  Lines           ?    19644           
  Branches        ?        0           
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  Hits            ?     9762           
  Misses          ?     9268           
  Partials        ?      614           

Flag	Coverage Δ
unittests	49.69% <100.00%> (?)

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

Impacted Files	Coverage Δ
scheduler/storage/storage.go	77.59% <100.00%> (ø)