data_sources: return an empty list for an unknown carddef (#42960)

tests/test_backoffice_pages.py

@@ -5848,6 +5848,17 @@ def test_studio_card_item_link(pub, studio):
     resp = resp.click('card plop')
     assert '<div class="value">plop</div>' in resp
 
+    # link to a unknown carddef
+    carddef2.fields = [
+        fields.ItemField(id='1', label='Test', type='item',
+            data_source={'type': 'carddef:unknown', 'value': ''}),
+    ]
+    carddef2.store()
+    app = login(get_app(pub))
+    resp = app.get('/backoffice/data/')
+    resp = resp.click('bar')
+    resp = resp.click('Add')  # no error
+
     # look without access rights
     carddef.backoffice_submission_roles = None
     carddef.workflow_roles = {'_editor': None}

wcs/data_sources.py

@@ -149,7 +149,10 @@ def get_structured_items(data_source, mode=None):
     if data_source.get('type') and data_source.get('type').startswith('carddef:'):
         # cards
         from wcs.carddef import CardDef
-        carddef = CardDef.get_by_urlname(data_source['type'][8:])
+        try:
+            carddef = CardDef.get_by_urlname(data_source['type'][8:])
+        except KeyError:
+            return []
         items = [x.get_data_source_structured_item()
                  for x in carddef.data_class().select()
                  if not x.is_draft()]