network: don't assume NMActiveConnection has a device

In practice this has been seen to fail:

    JS ERROR: TypeError: active.get_devices(...)[0] is undefined
    ensureActiveConnectionProps@resource:///org/gnome/shell/ui/status/network.js:73:22
    _getMainConnection@resource:///org/gnome/shell/ui/status/network.js:1791:13
    _syncMainConnection@resource:///org/gnome/shell/ui/status/network.js:1809:32

GNOME/gnome-shell#1406
4 jobs for !588 with wjt/1406-ensureactiveconnectionprops-erroneously-assumes-connections-have-at-least-one-device in 10 minutes and 40 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Review
passed #333405
check_commit_log

00:00:45

 
  Build
passed #333406
build

00:08:09

 
  Test
passed #333407
test

00:01:45

passed #333408
test-pot

00:01:45