oai: fix error on document type processing

* Closes #789.

Co-Authored-by: Bertrand Zuchuat <bertrand.zuchuat@rero.ch>

sonar/modules/documents/serializers/schemas/dc.py

@@ -249,11 +249,9 @@ def get_titles(self, obj):
 
     def get_types(self, obj):
         """Get types."""
-        if obj['metadata'].get('documentType'):
-            return [
-                'http://purl.org/coar/resource_type/{type}'.format(
-                    type=obj['metadata']['documentType'].split(':')[1])
-            ]
+        types = obj['metadata'].get('documentType', '').split(':')
+        if len(types) == 2:
+            return [f'http://purl.org/coar/resource_type/{types[1]}']
 
         return []