Check if extras exist before converting

In some situations like when validating an already validated dataset we
might be missing the `extras` key.

See
ckan/ckanext-dcat#305
ckan/ckan#7571

ckanext/scheming/plugins.py

@@ -279,7 +279,7 @@ def composite_convert_to(key, data, errors, context):
                 del data[(f,)]
 
         if action_type == 'show':
-            if composite_convert_fields:
+            if composite_convert_fields and data_dict.get("extras"):
                 for ex in data_dict['extras']:
                     if ex['key'] in composite_convert_fields:
                         data_dict[ex['key']] = json.loads(ex['value'])