NMDeviceWired: don't unconditionally access device.active_connection

NMDeviceWired: don't unconditionally access device.active_connection

The active_connection might be null, but the NMApplet code might
still belive the device wrapper is the primary connection (because
dbus signals are emitted one after the other).
(Also, remove an old and wrong comment about bluetooth in wwan code)

Fixes:

(gnome-shell:22511): Gjs-WARNING **: JS ERROR: Exception in callback for signal: icon-changed: TypeError: this._device.active_connection is null
NMDeviceWired<.getIndicatorIcon@resource:///org/gnome/shell/ui/status/network.js:475
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
NMApplet<._updateIcon@resource:///org/gnome/shell/ui/status/network.js:1790
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
_emit@resource:///org/gnome/gjs/modules/signals.js:124
NMConnectionSection<._addConnection/<@resource:///org/gnome/shell/ui/status/network.js:265
_emit@resource:///org/gnome/gjs/modules/signals.js:124
NMConnectionItem<._sync@resource:///org/gnome/shell/ui/status/network.js:137
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
NMConnectionItem<.setActiveConnection@resource:///org/gnome/shell/ui/status/network.js:169
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
NMConnectionDevice<._activeConnectionChanged@resource:///org/gnome/shell/ui/status/network.js:327
wrapper@resource:///org/gnome/gjs/modules/lang.js:169

https://bugzilla.gnome.org/show_bug.cgi?id=723570