Listingblock title returns None

The default title of the listingblock returns None.
Some packages (like collective.opengraph) can't handle this.

There is a default value '' defined in getDefaultTitle-Method:

```python
return registry.get('ftw.contentpage.listingblock.defaulttitle', '')
```

but this returns only '' if the key 'ftw.contentpage.listingblock.defaulttitle' does not exists. In our case, the key do exists with the value None. So the default value '' will be ignored.

It should be something like

```python
value = registry.get('ftw.contentpage.listingblock.defaulttitle', None)
return value and value or ''
```

The default title in the registry is set like this:

```xml
    <record name="ftw.contentpage.listingblock.defaulttitle">
        <field type="plone.registry.field.TextLine">
            <title>Default title of a listingblock</title>
            <required>False</required>
        </field>
        <value></value>
    </record>
```