Don't display legend if not provided

Allow for fieldsets to not have a legend:

```
self.helper.layout = Layout(
        Fieldset(
            None,
            'email', 
            'first_name',
            'last_name'
            ),
        Fieldset(
            _('Groups'),
            'group'
            ),
        Fieldset(
            _('Options'),
            'all_invoices'
            )
        )
```