to_latex shifts column names if MultiIndex contains empty label
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame([[1, 2]], columns=pd.MultiIndex.from_tuples([('', 'first'), ('2', 'second')]))

In [3]: df
Out[3]: 
             2
  first second
0     1      2

In [4]: print(df.to_latex())
\begin{tabular}{lrr}
\toprule
{} &      2 \\
{} & first & second \\
\midrule
0 &     1 &      2 \\
\bottomrule
\end{tabular}
```

#### Problem description

Notice that the headers line has 2 elements rather than 3.

#### Expected Output

``` python
\begin{tabular}{lrr}
\toprule
{} &    {} &      2 \\
{} & first & second \\
\midrule
0 &     1 &      2 \\
\bottomrule
\end{tabular}
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.23.0.dev0+422.g0bfb61b21
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.14.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1

</details>
