can nod concatenate character 'm' and a series
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame({"a": [1, 2, 3], })
print("m" + df["a"].astype(str))
```
#### Problem description
```
Traceback (most recent call last):
  File "D:/test/plot1.py", line 4, in <module>
    print("m" + df["a"].astype(str))
  File "C:\Python37\lib\site-packages\pandas\core\ops\__init__.py", line 1016, in wrapper
    assert np.isnat(right)
TypeError: ufunc 'isnat' is only defined for datetime and timedelta.
```
#### Expected Output
m1
m2
m3
#### 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 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.3
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : 2.6.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None

</details>
