Don't try to migrate defective documents.

This change hardens the migration to not fail on documents without
doucument version from a failed import ('Uninitialized document').

mayan/apps/documents/south_migrations/0027_move_filename_to_label.py

@@ -15,7 +15,8 @@ class Migration(DataMigration):
         # NOTE: This migration take a while. Maybe the use of F objects could
         # improve it.
         for document in orm.Document.objects.all():
-            document.label = document.versions.order_by('timestamp').last().filename
+            if document.versions.order_by('timestamp').last():
+                document.label = document.versions.order_by('timestamp').last().filename
             document.save()
 
     def backwards(self, orm):