keyboard: Use NULL rather than "" to disable keybindings

When disabling a keybinding, we set its value to { "", NULL } in gsettings
(bindings are stored as arrays of strings).
However, when a binding is disabled by default, its value is set to {
NULL }, not to the empty string.

The use of "" dates back to gconf where I think NULL was not a valid
value. Now that we have switched to gsettings, we can use NULL rather
than an artificial "".

https://bugzilla.gnome.org/show_bug.cgi?id=732383