Supposed way to have a login without password but still able to login via ssh?

[Federico Giannici]

I have a login that I want to be able to access only via ssh with a 
certificate (in ~/.ssh/authorized_keys).

So I have disabled the password ('*') but left a valid shell. Something 
like this in /etc/master.passwd:

mylogin:*:1001:1001::0:0:My login:/home/mylogin:/bin/sh


But in this way every day a receive a mail with the following:

Checking the /etc/master.passwd file:
Login mylogin is off but still has a valid shell and alternate access 
files in home directory are still readable.


What is the supposed way to define an account without a password but 
with a valid shell (to access via ssh with a certificate)?

Thanks.