Update RecordPubli to fix a bug in preprint_number.

modules/invenio_tools/recordpubli.py

@@ -657,7 +657,8 @@ class RecordPubli(Record):
         """The ArXiv preprint number.
 
         Returns:
-            str: empty string when it is not defined.
+            unicode:
+                empty string when it is not defined.
 
         """
         if u"primary_report_number" not in self:
@@ -666,7 +667,7 @@ class RecordPubli(Record):
         data = self[u"primary_report_number"]
         data = (data if isinstance(data, list) else [data])
 
-        li = [el for el in data if el.startswith(ARXIV)]
+        li = [el for el in data if el is not None and el.startswith(ARXIV)]
         if len(li) == 1:
                 return li[0]