990113: '500 Internal Server Error' importing manifest from stage

Was passing a value of empty string in the manifest for the field
Content.vendor. This got translated to null in the database column
and the field i non-null. Will use string 'unknown'. The data originates
too far away for us to control.
Merge pull request #339 from candlepin/wottop/990113

990113: '500 Internal Server Error' importing manifest from stag