to_html vertically expands multiindex cells if there are empty strings
Notice in the html below the second index column's 'a' field is given as `<th rowspan="2" valign="top">a</th>`, expanding over what should be an empty cell.

I actually found this using a pivot table, `margins=True`, as this creates a row keyed like `('All', '', '')`.

```
print df = pd.DataFrame({'c1': ['a', 'b'], 'c2': ['a', ''], 'data': [1, 2]}).set_index(['c1', 'c2'])
       data
c1 c2      
a  a      1
b         2

print df.to_html()
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th></th>
      <th>data</th>
    </tr>
    <tr>
      <th>c1</th>
      <th>c2</th>
      <th></th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>a</th>
      <th rowspan="2" valign="top">a</th>
      <td> 1</td>
    </tr>
    <tr>
      <th>b</th>
      <td> 2</td>
    </tr>
  </tbody>
</table>
```
