PAM - Pluggable Authentication Modules.
Someone was trying to log into your ssh server. As the username he used doesn't exist on your system, ssh passed to PAM "null handle". (A "handle", I think is something than should identify the user). PAM responds that the user with a null handle is "illegal" (non-existent), therefore the password fails and PAM refuses to authenticate it. sshd rejects the connection then.
Nothing to worry about, except that someone was trying to log in.