astype conversion to str converts NaN object to literal nan string

[joetl]

# Your code here
import pandas as pd
from numpy import nan
df = pd.DataFrame([{'sample': nan }])
df.iloc[0].sample
df.astype('str').iloc[0].sample

Problem description

When converting any datatype column to string using astype('str') null values (NaN object) is converted to literal nan string
Instead of remaining as null object value

[TomAugspurger]

Dupe of #25353, and probably others.