Dont store n/a creds
Don't user merge! because can modify self.compat
Land #2805 - Avoid using merge! which can modify self.compat

With the use of merge!, it can cause the user to unable to choose a
payload after switching from a different target selection.
Land #2792 - SSO/Mimikatz module overwrites password with N/A
Merge remote-tracking branch 'rapid7/master'