Revert "smb2: fix anonymous login"

This reverts commit 205963ad09.

According to git blame on libsmb2, anonymous login was always enabled
with a NULL password.

I don't know what happened when I tested this reverted commit, I may
have mix up VLC or/and smb/smb2/dsm builds.

For future reference, see libsmb2/lib/ntlmssp.c:
...
encode_ntlm_auth(...)
{
...
    if (auth_data->password == NULL) {
            anonymous = 1;
            goto encode;
    }
...
}

Fixes #27113

modules/access/smb2.c

@@ -691,8 +691,8 @@ vlc_smb2_connect_open_share(stream_t *access, const char *url,
     if (!username)
     {
         username = "Guest";
-        /* An empty password enable ntlmssp anonymous login */
-        password = "";
+        /* A NULL password enable ntlmssp anonymous login */
+        password = NULL;
     }
 
     smb2_set_security_mode(sys->smb2, SMB2_NEGOTIATE_SIGNING_ENABLED);