See #RM8838. Handle null domain_sid properly

This switches to the local sid if the domain sid is null, even if the ACTION is set to DOMAIN. This solves the issue identified in #RM8838 (https://dev.metasploit.com/redmine/issues/8838).

Note that this does not try to solve the remaining issues with the module (datastore[] assignments, etc). 

```
[*] 192.168.0.4 PIPE(LSARPC) LOCAL(NAS - 5-21-2272853860-1115691317-1341221697) DOMAIN(WORKGROUP - )
[-] 192.168.0.4 No domain SID identified, falling back to the local SID...
[*] 192.168.0.4 USER=guest RID=501
[*] 192.168.0.4 GROUP=None RID=513
```