Fix null password value on hash_user_password

My operation as bellows:

1. Run command "keystone help user-create", we can see the --pass parameter is optional

2. So i create a user without --pass

3. The error msg raised:
    "Expecting to find string in password. The server could not comply with the request since it is either malformed or otherwise incorrect. The client is assumed to be in error. (HTTP 400)"

4. Find that the keystoneclient post the request to server, the "password" is null:
     REQ BODY: {"user": {"email": null, "password": null, "enabled": true, "name": "hthtest", "tenantId": null}}

5. The server check the length of the password when hash it. So the TypeError be catched, and the server raised the exception.

6. So i think it's no need to hash the password if it's a null value.