Fix reduction agg with UDF

[codingl2k1]

What do these changes do?

Related issue number

Fixes https://github.com/xorbitsai/xorbits/issues/733

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass

[codecov[bot]]

Codecov Report

Merging #737 (3d39b25) into main (30cac74) will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #737      +/-   ##
==========================================
- Coverage   93.55%   93.54%   -0.01%     
==========================================
  Files        1026     1026              
  Lines       79586    79586              
  Branches    16497    16497              
==========================================
- Hits        74456    74449       -7     
  Misses       3450     3450              
- Partials     1680     1687       +7     

Flag	Coverage Δ
unittests	93.43% <ø> (-0.01%)	:arrow_down:

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

Files	Coverage Δ
python/xorbits/_mars/dataframe/reduction/sum.py	100.00% <ø> (ø)

... and 12 files with indirect coverage changes