Send empty password if missing

For telnet/ftp/... minipot, we check presence of both username and password data fields.

In some cases, there is missing password which raised errors such as:

Invalid message from client FOO

Both password and username must be present (empty, if needed) in login message type to make it processable on server-side.

Wrong message:

sentinel/collect/minipot: {
    'ts': 1597078735,
    'id': 'FOO',
    'device_token': 'barred',
    'type': 'ftp',
    'action': 'login',
    'data': {
        'username': 'admin'
    },
    'ip': '192.0.2.42'
}

Correct message:

sentinel/collect/minipot: {
    'ts': 1597078735,
    'id': 'FOO',
    'device_token': 'barred',
    'type': 'ftp',
    'action': 'login',
    'data': {
        'username': 'admin',
        'password': ''
    },
    'ip': '192.0.2.42'
}