Do not allow LDAP login with empty password

[satterly]

(venv) ➜  python-alerta-client git:(master) http -v :8080/auth/login username=foo password=
POST /auth/login HTTP/1.1
Accept: application/json, */*;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Length: 35
Content-Type: application/json
Host: localhost:8080
User-Agent: HTTPie/2.1.0

{
    "username": "foo",
    "password": ""
}

HTTP/1.0 401 UNAUTHORIZED
Access-Control-Allow-Origin: http://localhost
Content-Length: 131
Content-Type: application/json
Date: Sat, 24 Oct 2020 20:31:10 GMT
Server: Werkzeug/1.0.1 Python/3.7.3
Vary: Origin
X-Request-ID: 06eefaf8-1716-49de-998f-e3c05f59292e

{
    "code": 401,
    "errors": null,
    "message": "password not allowed to be empty",
    "requestId": null,
    "status": "error"
}

Fixes #1277