settings: make `option_entries` NULL-terminated

As stated in the documentation of gtk_init_with_args (to which option_entries is passed):

The array must be NULL-terminated.

(source: https://docs.gtk.org/gtk3/func.init_with_args.html)

On Chimera Linux, this was causing thunar-volman-settings to segfault when literally any argument was passed to it, whether an option or not.