config: Fall back to a default submission nick if lookup fails

3 jobs for configure-moz in 8 minutes and 33 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #685279
ubuntu-16.04

00:02:15

passed #685280
ubuntu-18.04

00:08:23

passed #685281
ubuntu-18.04-no-backend

00:02:00