network: Don't assume the active connection has been processed first

network: Don't assume the active connection has been processed first

`NMConnectionDevice._sync()` is responsible for setting up the active
connection that we'll end up displaying. It expects the active
connection to already be in a map `_connectionItems`. If it isn't in
there, we get a null dereference and the indicator can get into a weird
state where it doesn't display devices / connections properly.

Let's change this expectation. If there is an active connection,
`_deviceAdded()` will eventually get to it and call `_sync()` to set up
the active connection state. We make `_sync()` tolerate there being no
active connection when it's called.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/140