assistant: Validate server address

The URI needs to have at least a scheme and a server address.

Fixes #58
2 jobs for fix-add-connection-issues in 2 minutes and 58 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed flatpak #1399376
flatpak

00:02:36

 
  Fedora
passed fedora:latest #1399377

00:02:58