vulnerability bugs version 6

The PR fixes a number of bugs outlined in the GEM Vulnerability Database & GUI Google doc.
Looking ```openquakeplatform/vulnerability/models.py``` 
```
    structure_type = models.IntegerField(
        choices=STRUCTURE_TYPES, blank=True, null=True,
        help_text='"Building" type is only applicable to "Structure specific"'
        ' or "Structure class"')
```
Currently all curves are with this value set but the field is potentially ```nullable``` so could be a good choice to check if it exists.