Add support to Null Session for SMBv2 and SMBv3

[cdelafuente-r7]

I noticed guest or anonymous access doesn't work with SMB2/3. This kind of access is not enabled by default on Windows, so I'm not sure if it would be super useful.

Testing against Windows Server 2019 with guest access enabled and SMBv1 disabled (see this documentation to enable guest access):

• on Windows:

  C:\>net use \\192.168.2.44\IPC$ /user:"" ""
  The command completed successfully.

• with RubySMB:

  ❯ ruby examples/tree_connect.rb 192.168.2.44 "" "" IPC$ 2
  SMB2 : (0xc000006d) STATUS_LOGON_FAILURE: The attempted logon is invalid. This is either due to a bad username or authentication information.
  Failed to connect to \\192.168.144.224\IPC$: The server responded with an unexpected status code: STATUS_USER_SESSION_DELETED

[zeroSteiner]

Is this still a problem since #190 was closed?

[cdelafuente-r7]

Yes, absolutely. I retested this use case and it works great now. Thanks!