How to pentest an IPC$ NULL session ?
You could use these command line and check if the result is successful. If a connection is established, you know that you could enumerate informations about a Microsoft system with rpcclient, enum.exe, superscan.exe, nmap, …
From Windows :
net use N: \\172.16.200.146\IPC$ /u:"" ""
From UNIX/Linux :
smbclient -I 172.16.200.146 -L domain.com -U -N