BUG: Replace methods fills value from previous row when replacing with None
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
import numpy as np
import pandas as pd

df = pd.DataFrame({'x': [10, 20, np.nan], 'y': [30, 40, 50]})
print(df.replace(np.NaN, None))

#       x   y
# 0  10.0  30
# 1  20.0  40
# 2  20.0  50
```

#### Problem description

When replacing NaN with None using replace method, value, NaN value is replaced with value from previous row instead of None. 

#### Expected Output
```
#       x   y
# 0  10.0  30
# 1  20.0  40
# 2  None  50
```

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : None.None
pandas           : 1.0.0rc0
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : 0.8.6
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```
</details>
