use explicit blank values

when searching for BlankUsername or BlankPassword in
the database. prevents wierd nil validations errors
qucik spec to cover that case

added a spec to cover the blankpassword case
to make sure we create them correctly now

MSP-11592
Merge pull request #78 from rapid7/bug/MSP-11592/blank-password-failure

Bug/msp 11592/blank password failure