read_excel error for header=None and index_col as list
#### Code Sample

```python
import pandas as pd
df = pd.read_excel('Book1.xlsx', header=None, index_col = [0, 1])

```
#### Problem description

Above leads to the following error. This seems to be related to the combination of header=None and index_col as a list. If I give an integer index_col, or leave out header=None (or specify a header) there is no error.

The only other reference to this issue I found was [this stack overflow question](https://stackoverflow.com/questions/48334347/read-excel-in-pandas-giving-error-for-no-header-and-multiple-index-cols).

I am seeing this in the latest release (1.0.1) as well as an older one (0.25.3). I haven't been able to install from master because I'm working on windows and don't have a compiler set up.

Example exel file (although the contents doesn't matter, it could be workbook with an empty sheet): [Book1.xlsx](https://github.com/pandas-dev/pandas/files/4171589/Book1.xlsx)


```
<redact>\lib\site-packages\pandas\io\excel\_base.py in read_excel(io, sheet_name, header, names, index_col, usecols, squeeze, dtype, engine, converters, true_values, false_values, skiprows, nrows, na_values, keep_default_na, verbose, parse_dates, date_parser, thousands, comment, skipfooter, convert_float, mangle_dupe_cols, **kwds)
    332         convert_float=convert_float,
    333         mangle_dupe_cols=mangle_dupe_cols,
--> 334         **kwds,
    335     )
    336

<redact>\lib\site-packages\pandas\io\excel\_base.py in parse(self, sheet_name, header, names, index_col, usecols, squeeze, converters, true_values, false_values, skiprows, nrows, na_values, parse_dates, date_parser, thousands, comment, skipfooter, convert_float, mangle_dupe_cols, **kwds)
    883             convert_float=convert_float,
    884             mangle_dupe_cols=mangle_dupe_cols,
--> 885             **kwds,
    886         )
    887

<redact>\lib\site-packages\pandas\io\excel\_base.py in parse(self, sheet_name, header, names, index_col, usecols, squeeze, dtype, true_values, false_values, skiprows, nrows, na_values, verbose, parse_dates, date_parser, thousands, comment, skipfooter, convert_float, mangle_dupe_cols, **kwds)
    467                 # Forward fill values for MultiIndex index.
    468                 if not is_list_like(header):
--> 469                     offset = 1 + header
    470                 else:
    471                     offset = 1 + max(header)

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
```


#### Expected Output

A DataFrame with two column indexes.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
