BUG: Fix `_generate_value` when building mock dataframe

[aresnow1]

What do these changes do?

Fix _generate_value when building mock dataframe.

Related issue number

Fixes #637

Check code requirements

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

[codecov[bot]]

Codecov Report

Merging #645 (4b91be6) into main (5eab408) will decrease coverage by 6.96%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
- Coverage   93.60%   86.65%   -6.96%     
==========================================
  Files        1024     1024              
  Lines       79430    79432       +2     
  Branches    16474    16475       +1     
==========================================
- Hits        74353    68833    -5520     
- Misses       3407     9058    +5651     
+ Partials     1670     1541     -129     

Flag	Coverage Δ
unittests	86.55% <100.00%> (-6.95%)	:arrow_down:

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Files Changed	Coverage Δ
python/xorbits/_mars/dataframe/utils.py	95.44% <100.00%> (+0.27%)	:arrow_up:

... and 110 files with indirect coverage changes