Null in facet means missing ?

I'm not sure if this depends on the Solr version, but in my case, missing and null values are the same.

A facet query return an item with a null name depending on the facet.missing parameter.

```
<lst name="inspireAnnex">
<int name="ii">51</int>
<int name="i">26</int>
<int name="iii">21</int>
<int>48</int>
</lst>
```


Currently, the missing are added based on the total minus the total of items and a null category is displayed on the charts:
![image](https://cloud.githubusercontent.com/assets/1701393/5440151/41a3f278-8486-11e4-8f2a-cb8058736920.png)


When the missing option is on, we should probably use the facet.missing parameter and rely on the response content for the no value item ?
![image](https://cloud.githubusercontent.com/assets/1701393/5440159/4f7b9fb8-8486-11e4-925e-35e562907f56.png)