fix: can not create empty knowledge

[laipz8200]

(cherry picked from commit f1ba9cf)

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Summary

Screenshots

Before	After
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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.94%. Comparing base (f4c02e4) to head (fc9e5aa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #36336      +/-   ##
==========================================
- Coverage   85.94%   85.94%   -0.01%     
==========================================
  Files        4454     4454              
  Lines      211503   211503              
  Branches    39515    39515              
==========================================
- Hits       181782   181779       -3     
- Misses      26509    26512       +3     
  Partials     3212     3212              

Flag	Coverage Δ
dify-ui	94.59% <ø> (ø)
web	86.55% <ø> (-0.01%)	⬇️

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[CodingOnStar]

LGTM