Set empty password for a specific user and user can login without password

Issue

• Have a nullok entry in /etc/pam.d/system-auth and /etc/pam.d/password-auth file:

$ more /etc/pam.d/system-auth |grep nullok
auth [success=1 default=ignore] pam_succeed_if.so user <username> 
auth sufficient pam_unix.so try_first_pass nullok 

• If an enabled account on the system is not password protected, then the users can login with no password. How do fix this issue?