datatype converstion(.astype) error for bool type.
Datatype converstion(.astype) error for bool type.

```python
import pandas as pd
pd.__version__

df_1 = pd.DataFrame({'c_bool': [None, None, True, True, True,False]})
print(df_1['c_bool'].dtypes)
df_1['c_bool'] = df_1['c_bool'].astype(bool)
print(df_1['c_bool'].dtypes)
df_1

# '0.24.2'
# object
# bool
#   c_bool
# 0   False
# 1   False
# 2    True
# 3    True
# 4    True
# 5   False
```
I have a column that has some None/True/False values. Initially, the datatype of that column is 'object'. After I convert it to bool as datatype, Null values are converting to False. Which is not expected behavior.