Fix error when create user with LDAP backend

Creating users would fail when using the LDAP backend. The error
from the server was '400 Bad Request' with the message
'must be string, not None'.

The fix is to filter out 'None' attr names when creating a user.

Closes-Bug: #1231772

Change-Id: I747f39039b9c840a3e5861b66996dee328510cd6