[TST] Revise test_astype_str_map #28454
Description
follow up #28176 (review)
Right now, astype(str) converts np.nan into string.
In [3]: pd.Series(['foo', np.nan]).astype(str)
Out[3]:
0 foo
1 nan
dtype: object
After merging #28176, astype(str) will preserve np.nan.
Review test_astype_str_map in pandas/tests/series/test_dtypes.py to get rid of any skipna keyword argument.