BUG: `multicolumn` of `to_latex` does not work correctly when `index==False` 
### 

- [X] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

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


### Reproducible Example

```python
import pandas as pd
df = pd.DataFrame([[1,2,3,4]], columns=pd.MultiIndex.from_tuples([('1', 'a'), ('1', 'b'), ('2', 'a'), ('2', 'b'), ]))
print(df.to_latex(index=False, multicolumn=True,))

\begin{tabular}{rrrr}
\toprule
1 & \multicolumn{2}{l}{2} \\
a & b & a & b \\
\midrule
1 & 2 & 3 & 4 \\
\bottomrule
\end{tabular}
```


### Issue Description

The top level of multicolumns are not located correctly.

### Expected Behavior

\begin{tabular}{rrrr}
\toprule
\multicolumn{2}{l}{1} & \multicolumn{2}{l}{2} \\
a & b & a & b \\
\midrule
1 & 2 & 3 & 4 \\
\bottomrule
\end{tabular}


### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.8.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.6.0
Version          : Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.1
numpy            : 1.21.1
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.2
setuptools       : 57.4.0
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.25.0
pandas_datareader: 0.10.0
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.7.0
sqlalchemy       : None
tables           : 3.6.1
tabulate         : 0.8.9
xarray           : 0.19.0
xlrd             : 2.0.1
xlwt             : None
numba            : 0.53.1

</details>
