DataFrame.replace fails to replace value when columns are specified and only non-replacement columns contain pd.NA
#### Code Sample, a copy-pastable example if possible

```python
In [87]: df2 = pd.DataFrame([['a', 1], ['b', pd.NA]])

In [108]: df2
Out[108]: 
   0     1
0  a     1
1  b  <NA>

In [109]: df2.replace({0: 'a'}, np.nan)
/home/torstein/anaconda3/lib/python3.7/site-packages/pandas/core/missing.py:47: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  mask = arr == x
Out[111]: 
   0     1
0  a     1
1  b  <NA>

In [112]: df2.dtypes
Out[112]: 
0    object
1    object
dtype: object

# This works:
In [121]: df2[0].replace('a', np.nan)
Out[121]: 
0    NaN
1      b
Name: 0, dtype: object

# This also works:
In [126]: pd.DataFrame([['a', 1], ['b', np.nan]]).replace({0: 'a'}, 'b')
Out[126]: 
   0    1
0  b  1.0
1  b  NaN


```
#### Problem description

There are multiple similar issues, but in this case, there are no NaNs in the column specified by the replacement-dictionary. If the dataframe is created without additional column `1` with a `pd.NA`, even though no replacement is performed on that column, then replace works.

#### Expected Output

'a' to be replaced with nan
#### Output of ``pd.show_versions()``

<details>
pandas           : 1.0.2
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 41.6.0.post20191030
Cython           : 0.29.13
pytest           : 5.2.2
hypothesis       : None
sphinx           : 2.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.2
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 2.2.3
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.2.2
pyxlsb           : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.10
tables           : 3.5.2
tabulate         : 0.8.5
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.2
numba            : 0.46.0

</details>
