Fix critical when setting a NULL label

st_label_set_text() does not accept NULL, and emits a g_critical
in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=654638

js/ui/status/network.js

@@ -674,7 +674,7 @@ NMDevice.prototype = {
         let dev_product = this.device.get_product();
         let dev_vendor = this.device.get_vendor();
         if (!dev_product || !dev_vendor)
-	    return null;
+	    return '';
 
         let product = Util.fixupPCIDescription(dev_product);
         let vendor = Util.fixupPCIDescription(dev_vendor);