keyboard: Do not load xkb-options from localed when explicitly set to empty

If a user on a system with non-empty xkb-options in localed decides to disable the options locally by setting the GSettings key to an empty array, the value will be reset back to localed value on next log-in.

Let’s stop overriding xkb-options if the GSettings key is already set.

A consequence of this patch is that when an administrator changes the previously empty xkb-options in localed, users that had already logged in before the change will keep the empty option list. But one would likewise not expect create_initial_settings to update the GSettings key on change from a non-empty xkb-options so this patch just makes the behaviour more consistent.