Problem with DataFrame.replace using None
#### Code Sample, a copy-pastable example if possible

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

ar = np.random.normal(size=[100,10])
df = pd.DataFrame(ar).astype(str)

df.replace('0',None)
```
#### Problem description

Every time I run something similar to this (replacing, in a string DataFrame/DataFrame column, a specific string with `None`), the columns get filled with the element directly above it in the column; this creates a lot of confusion since None is, pretty much, THE "pythonic" way of signalling the absence of a variable/whatever. It works fine if replaced with np.nan, but if anything, I would expect them to work very approximately the same.

#### Expected Output

The expected output would be all strings matching the expression in `replace` to either be replaced with either `None` or `NaN`, or at least a warning message alerting for this behaviour.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Darwin
OS-release: 18.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.1
pytest: None
pip: 19.0.3
setuptools: 40.6.2
Cython: None
numpy: 1.16.0
scipy: 1.2.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: 4.7.1
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
