Fix #4304 - Blank password is tried when it shouldn't happen

Fix #4304

## Verification ##

- [ ] echo username > /tmp/users.txt
- [ ] Set up a Windows box
- [ ] Start msfconsole
- [ ] ```use auxiliary/scanner/smb/smb_login```
- [ ] ```set rhosts [IP]```
- [ ] ```set user_as_pass true```
- [ ] ```set user_file /tmp/users.txt```
- [ ] Make sure blank_password is false
- [ ] ```run```
- [ ] You should not see an empty password being used (basically there should be only one login attempt)