Fix IndexError when mail is set to an empty list

IndexError raised due to:
{'cn': 'Alessio Curri (new)', 'sAMAccountName': 'alessiocurri2', 'mail': []}

JIRA INFRA-610 #action In Progress

app/utils.py

@@ -111,7 +111,10 @@ def attribute_to_string(value):
     :returns: string
     """
     if isinstance(value, list):
-        return value[0]
+        try:
+            return value[0]
+        except IndexError:
+            return ""
     else:
         return value