wallpaper: Stop passing NULL as the options in the request's response

xdp_request_emit_response expects a valid GVariant that corresponds to
the initial format passed when starting first creating the request.

Since NULL isn't a valid GVariant, it made the program crash with the
following backtrace.

So this commit passes back the options GVariant in the request's
response, as expected by xdp_request_emit_response.

Backtrace:

(gdb) bt full
   from /lib64/libglib-2.0.so.0
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   from /lib64/libglib-2.0.so.0
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    arg_response=2, arg_results=0x0) at src/request.c:44
        request = 0x7fffd0008750
        skeleton = 0x7fffd0008750
        connections = 0x7fffd4023c60
--Type <RET> for more, q to quit, c to continue without paging--
        l = <optimized out>
        signal_variant = <optimized out>
No symbol table info available.
No symbol table info available.
   from /lib64/libgobject-2.0.so.0
No symbol table info available.
No symbol table info available.
   from /lib64/libgobject-2.0.so.0
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   from /lib64/libgobject-2.0.so.0
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   from /lib64/libgobject-2.0.so.0
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    arg_response=<optimized out>, arg_results=<optimized out>)
    at src/xdp-dbus.c:31794
--Type <RET> for more, q to quit, c to continue without paging--
No locals.
    task=<optimized out>, source_object=<optimized out>,
    task_data=0x7fffd0008750, cancellable=<optimized out>)
    at src/wallpaper.c:241
        path = 0x0
        request = 0x7fffd0008750
        parent_window = 0x7fffd800a290 "wayland:&Bwwe~S\"tvZ.#NlVx3.jCD ;NpEiav\"r"
        app_id = 0x7fffdc01ee70 "org.gnome.Epiphany.Devel"
        error = 0x0
        uri = 0x0
        opt_builder = {u = {s = {partial_magic = 140737353935335,
              type = 0x28245187, y = {140737018588464, 93824993519896, 1,
                140737348105957, 80, 93824993519880, 93824993519872, 39,
                140736817273504, 140737349883604, 0, 0, 673468807,
                140736548841120}}, x = {140737353935335, 673468807,
              140737018588464, 93824993519896, 1, 140737348105957, 80,
              93824993519880, 93824993519872, 39, 140736817273504,
              140737349883604, 0, 0, 673468807, 140736548841120}}}
        impl_request = 0x0
        options = 0x7fffdc005730
        show_preview = 1
--Type <RET> for more, q to quit, c to continue without paging--
        fd = <optimized out>
        permission = <optimized out>
        request_auto_unlock136 = 0x7fffd0008790
   from /lib64/libgio-2.0.so.0
No symbol table info available.
   from /lib64/libglib-2.0.so.0
No symbol table info available.
No symbol table info available.
No symbol table info available.
No symbol table info available.