BUG: str.cat produces NaNs when others is an Index

[brandon-b-miller]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

import pandas as pd
print(pd.__version__)

sr = pd.Series(['a','b','c','d','e'])
others = pd.Index(['a','b','c','d','e'])

result = sr.str.cat(others=others)
print(result)

1.0.3
0    NaN
1    NaN
2    NaN
3    NaN
4    NaN
dtype: object

Problem description

The result should be the same as when others is a list or numpy array with the same values. The result is correct for pandas < 1.0.

Expected Output

0    aa
1    bb
2    cc
3    dd
4    ee
dtype: object

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.4.0-134-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.3
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 46.1.1.post20200322
Cython : 0.29.15
pytest : 5.4.1
hypothesis : 5.7.0
sphinx : 2.4.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.13.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 0.15.0
pytables : None
pytest : 5.4.1
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : 0.48.0