BUG: `map_chunk` wrong results when input is empty data

[ChengjieLi28]

What do these changes do?

Related issue number

Fixes #723

Check code requirements

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

[codecov[bot]]

Codecov Report

Merging #727 (9cb4f26) into main (b320ca3) will decrease coverage by 12.01%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             main     #727       +/-   ##
===========================================
- Coverage   61.64%   49.64%   -12.01%     
===========================================
  Files        1058     1058               
  Lines       79780    79775        -5     
  Branches    16504    12431     -4073     
===========================================
- Hits        49181    39604     -9577     
- Misses      28045    38581    +10536     
+ Partials     2554     1590      -964     

Flag	Coverage Δ
unittests	49.63% <0.00%> (-11.94%)	:arrow_down:

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

Files	Coverage Δ
python/xorbits/_mars/dataframe/base/map_chunk.py	19.48% <0.00%> (-69.20%)	:arrow_down:

... and 346 files with indirect coverage changes