BUG: Incorrect parsing csv header line if no separator in the end
- [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.

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

---

```python

import pandas as pd
from io import StringIO

data = """
"date_time";"T";"Po"
"04.05.2021 09:00";"9.8";"759.0";
"""

df = pd.read_csv(StringIO(data), sep=';',index_col=0)

df['T']

df.head(1)

```

#### Problem description

When there is no separator at the end of the header line, pandas seems like skip first column (T) and shows second column instead.

```python

df['T']
df.loc[:,'T']

```
**Output:**
```
04.05.2021 09:00    759.0
Name: T, dtype: float64
```

**Expected:**
 (as df.iloc[0,0] give 9.8):
```
Местное время в Омске / им. Д. М. Карбышева (аэропорт)
04.05.2021 09:00    9.8
Name: T, dtype: float64
```
After I add ";" separator after "sss" bug disappeared, df.iloc[0,0] output is same in both case.


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

<details>

INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.6.8.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.21354
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.5
numpy            : 1.17.4
pytz             : 2018.9
dateutil         : 2.8.1
pip              : 19.0.3
setuptools       : 42.0.1
Cython           : 0.29.21
pytest           : 6.2.3
hypothesis       : None
sphinx           : 3.5.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.5.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : 1.3.1
fsspec           : 0.6.1
fastparquet      : None
gcsfs            : 0.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : 2.6.1
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.2.18
tables           : None
tabulate         : None
xarray           : 0.14.1
xlrd             : 1.2.0
xlwt             : None
numba            : 0.48.0

</details>
