[xfce4-keyboard-settings] "system defaults" warning is shown at startup if XkbDisable is unset

I'm using xfce4-settings 4.16.1 with d7541bca applied on top. I have created !58 (merged) as a possible fix.

xfce4-keyboard-settings in a new Xfce 4.16 installation displays the following warning at startup:

Likely related to #310 (closed). In existing installations, this is reproducible by resetting the property first:

$ xfconf-query -c keyboard-layout -p /Default/XkbDisable -r
$ xfce4-keyboard-settings