BUG: build_mock_groupby raises TypeError: 'int' object is not iterable

[codingl2k1]

Describe the bug

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To Reproduce

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>>> pdf.groupby("max_stars_repo_licenses")["max_issues_count"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/xorbits/python/xorbits/core/adapter.py", line 294, in wrapped
    return wrap_mars_callable(
  File "/home/ubuntu/xorbits/python/xorbits/core/adapter.py", line 472, in wrapped
    return from_mars(c(*to_mars(args), **to_mars(kwargs)))
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/groupby/getitem.py", line 138, in df_groupby_getitem
    return op(df_groupby)
  File "/home/ubuntu/xorbits/python/xorbits/_mars/core/mode.py", line 78, in _inner
    return func(*args, **kwargs)
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/groupby/getitem.py", line 49, in __call__
    indexed = groupby.op.build_mock_groupby()[self.selection]
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/groupby/core.py", line 149, in build_mock_groupby
    mock_obj = build_df(
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/utils.py", line 650, in build_df
    record = [[_generate_value(dtype, fill_value) for dtype in dtypes]] * size
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/utils.py", line 650, in <listcomp>
    record = [[_generate_value(dtype, fill_value) for dtype in dtypes]] * size
  File "/home/ubuntu/xorbits/python/xorbits/_mars/dataframe/utils.py", line 613, in _generate_value
    return convert(fill_value)
TypeError: 'int' object is not iterable

1. Your Python version
2. The version of Xorbits you use
3. Versions of crucial packages, such as numpy, scipy and pandas
4. Full stack of the error.
5. Minimized code to reproduce the error.

Expected behavior

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Additional context

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