Remove the special case for empty dataframes.
Fixes issue #6062.

can post a before and after of an empty frame (and also something with rows to make sure it hasn't changed)
 Before:

```
print(pd.DataFrame(columns=["asdf", "bsdf"]).to_html())
```

leads to

<table border="1" class="dataframe">
      <tbody>
        <tr>
          <td>Index([], dtype='object')</td>
          <td>Empty DataFrame</td>
        </tr>
      </tbody>
</table>

With the patch you get:

<table border="1" class="dataframe">
      <thead>
        <tr style="text-align: right;">
          <th></th>
          <th>asdf</th>
          <th>bsdf</th>
        </tr>
      </thead>
      <tbody>
      </tbody>
</table>

A dataframe with values (`pd.DataFrame(columns=["asdf", "bsdf"], data=[[1,2]])`) leads to

<table border="1" class="dataframe">
      <thead>
        <tr style="text-align: right;">
          <th></th>
          <th>asdf</th>
          <th>bsdf</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <th>0</th>
          <td> 1</td>
          <td> 2</td>
        </tr>
      </tbody>
</table>

in both cases.
 @filmor no what I mean is actually put up a picture, like a png/jpeg of what it looks like in the browser. You can execute the code and save it that way.
 can you add a release note (and a note in v0.14.0.txt for this), referencing the issue? (put in the API section)

looks good otherwise
 Done.
 @hayd @jorisvandenbossche look ok?
 Is this also how it will be displayed when printing a DataFrame in the notebook?
 It is, yes. If this is not desired I'd vote for fixing this in IPython, not in `df.to_html`. It makes a lot of sense to have `df.to_html` be parseable again.
 thanks
