BUG: String series cat produces NaNs when left operand has a custom index
- [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.

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

---

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

```python
import pandas as pd
print(pd.__version__)

x = pd.Series(['a','b','c','d','e'])
x.index = ['1', '2', '3', '4', '5']
pd_others = pd.Series(['f','g','h','i','j'])

print(x.str.cat(others=pd_others))

1.1.0.dev0+1690.g70d7c04ff
1    NaN
2    NaN
3    NaN
4    NaN
5    NaN
dtype: object
```

#### Problem description
Possibly related to https://github.com/pandas-dev/pandas/issues/33425. The trigger for this bug seems to be the custom index. I get the correct answer if the line where `x.index` is set is removed.  

#### Expected Output

```
1.1.0.dev0+1690.g70d7c04ff
0    af
1    bg
2    ch
3    di
4    ej
dtype: object
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 70d7c04ff585de361622e4fe1788480a7a4526b5
python           : 3.8.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
Version          : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0.dev0+1690.g70d7c04ff
numpy            : 1.18.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.4.0.post20200518
Cython           : 0.29.17
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
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
