mail: use scanner category if there's no association yet

welco/sources/mail/models.py

@@ -99,6 +99,8 @@ class Mail(models.Model):
         for association in self.associations.all():
             if association.formdef_category:
                 categories[association.formdef_category] = True
+        if not categories and self.scanner_category:
+            categories[self.scanner_category] = True
         if not categories:
             return '-'
         return categories.keys()