BUG: latex css conversion does not work on new applymap_index
### 

- [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.

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


### Reproducible Example

```python
df = DataFrame([[0], [1]], index=[0, 1])
styler = df.style
styler.applymap(lambda x: "color: red;" if x == 0 else "")
styler.applymap_index(lambda x: "color: blue;" if x == 0 else "")
print(styler.to_latex(convert_css=True))

\begin{tabular}{lr}
 & 0 \\
\colorblue 0 & \color{red} 0 \\
1 & 1 \\
\end{tabular}
```


### Issue Description

the new method applymap_index has not been hooked up to the CSS conversion for styler.to_latex.

### Expected Behavior

should convert to `color{blue}`

### Installed Versions

master