Catalog items with an empty hst:iconpath property should render a default icon

Catalog items with an empty 'hst:iconpath' property will render a broken image in the components toolbar in the template composer. Those catalog items should render a default icon instead.

Steps to reproduce:
1. Start the archetype
2. In the console, set the property /hst:hst/hst:configurations/myhippoproject/hst:catalog/myhippoproject-catalog/list/hst:iconpath to an empty string
3. In the CMS, open the "My Hippo Project" channel
4. Click 'Edit'
5. Click 'Add Components'

Expected: the 'List' component gets a default icon
Actual: the browser renders a 'broken image' placeholder for the 'List' component