x11: fix xkb_x11_keymap_new_from_device failing when a level name is empty

x11: fix xkb_x11_keymap_new_from_device failing when a level name is empty

The numpad:mac option doesn't specify a name for the first level:

    // On Mac keypads, level 1 and 2 are swapped.
    partial xkb_types "mac" {
        type "KEYPAD" {
            modifiers = None;
            map[None] = Level2;
            level_name[Level2] = "Number";
        };
        include "extra(keypad)"
    };

This means the atom for level name is XCB_ATOM_NONE. We tried to get its
name, which fails. This regressed in 40c00b472144d1684d2fb97cafef39.

Instead, translate it to XKB_ATOM_NONE, same as the previous behavior.

Fixes: https://github.com/xkbcommon/libxkbcommon/issues/229
Signed-off-by: Ran Benita <ran@unusedvar.com>