linux/amd64: fix unhandled promise at app startup without db connection

[misteveus]

When the app is installed via .deb an unhandled promise occurs at startup, if there is no database connection, instead of opening the create db connection prompt.

Unhandled Promise Rejection
TypeError: Cannot read property 'username' of undefined
    at StigDB.changeConfig (/usr/lib/stig/resources/app/src/db/db.ts:115:64)
    at new StigDB (/usr/lib/stig/resources/app/src/db/db.ts:98:18)
    at HTMLDocument.document.addEventListener (/usr/lib/stig/resources/app/src/main.ts:64:24)

The issue appears to come from checking the wrong property in the StigDB constructor.