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

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