BUG: read_csv - comment argument does not behave as expected
#### Code Sample, a copy-pastable example
Giving the sample file:
```text
#empty
a,b,c
1,2,3
2,2,3
3,3,3
```
I want to skip text line starting with character '3':
```python
pd.read_csv(filepath, sep=',', comment='3', header=1)
```

#### Problem description
In the resulting dataframe the line starting with char '3' is skipped, but also other occurrences of '3' are replaced by nan's.
```text
  a b c
0 1 2 nan
1 2 2 nan
```
#### Expected Output
I expect only record starting with the comment char is missing.
```text
  a b c
0 1 2 3
1 2 2 3
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.8.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-136-generic
Version          : #140-Ubuntu SMP Thu Jan 28 05:20:47 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.3
numpy            : 1.19.0
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 53.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : 1.3.23
tables           : None
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : 0.50.0


</details>
