Login impossible with account with no password

[Laurent CHARLES]

Hello,

I need your help to understand what is going on with an account with
no password.

- Le account is a regular user account with default configuration.
- Le password has been suppressed with a simple 'sudo passwd -d <logname>'

- Login on a console works well. We only need the logname to get in.
/var/log/auth.log shows "login[7705]: (pam_unix)
session opened" and "login[7705]: (pam_unix) session closed", when we logout.

But:
- It is not possible to do 'su <logname>'. The system requests a
"Password:", and [entrer] only a shows a "Sorry".
We can see in  /var/log/auth.log:
 su[10403]: (pam_securetty) access denied: tty 'pts/0' is not secure !
 su[10403]: (pam_unix) authentication failure;
 su[10403]: pam_authenticate: Authentication failure
- The same symptoms occur with a login from gdm

Question 1: Why my 'tty' is not "secure"?
Question 2: What is the actual difference (technically) between the
identification/login on a console and the one with 'su -' or with gdm?
Question 3: How to "open" a login with no password

Thank you for your help
Regards

--
-- Laurent