fix(source): fix warning from source manager when drop materialized source

[shanicky]

Signed-off-by: Peng Chen peng@singularity-data.com

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What's changed and what's your intention?

as title

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

#3473

[tabVersion]

lgtm

[codecov]

Codecov Report

Merging #3480 (80f3f7f) into main (726fc3c) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3480      +/-   ##
==========================================
- Coverage   74.40%   74.40%   -0.01%     
==========================================
  Files         769      769              
  Lines      107303   107304       +1     
==========================================
  Hits        79841    79841              
- Misses      27462    27463       +1     

Flag	Coverage Δ
rust	74.40% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/meta/src/rpc/service/ddl_service.rs	3.48% <0.00%> (ø)
src/meta/src/stream/source_manager.rs	30.60% <0.00%> (-0.07%)	⬇️

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