Inconsistent behavior when DataFrame with strings and None is created from lists or dictionary. 
#### Code Sample

```python
import pandas as pd


pd.__version__

# df created from list, None is casted to str
df = pd.DataFrame(["1", "2", None], columns=["a"], dtype="str")
type(df.loc[2].values[0])


# Equivalent df created from dict, None remains NoneType
df = pd.DataFrame({"a": ["1", "2", None]}, dtype="str")
type(df.loc[2].values[0]))

```
#### Problem description

None is not casted consistently for DataFrames with None values and dtype set to str.

If DataFrame is created from list, then None is casted to str -> None -> "None".
If DataFrame is created from dict, then None remains NoneType -> None -> None.

IMO, the latter is the preferred behavior. I hope you consider this when continuing your work on string types and na types in future versions.  


#### Expected Output

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 41.2.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
