`series.str.cat(series.str)` is concatenating only the largest string
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

arr = ["AbC", "de", "FGHI", "j", "kLLLm"]

ps = pd.Series(arr)
expect = ps.str.cat(others=ps.str)
print(expect)
Out[16]:
0           NaN
1           NaN
2           NaN
3           NaN
4    kLLLmkLLLm
dtype: object
```

#### Problem description
`series.str.cat(series.str)` is concatenating only the largest string in the series but it should concatenate all the strings element wise.

#### Expected Output
```python
Out[18]:
0        AbCAbC
1          dede
2      FGHIFGHI
3            jj
4    kLLLmkLLLm
dtype: object
```


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

<details>

In [2]: pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-862.14.4.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: None
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 5.1.2
pip: 19.2.3
setuptools: 41.2.0
Cython: 0.29.13
numpy: 1.17.1
scipy: None
pyarrow: 0.14.1
xarray: None
IPython: 7.8.0
sphinx: 2.2.0
patsy: None
dateutil: 2.8.0
pytz: 2019.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: 0.3.4
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
