Not accepting keycode 0 from virtual keyboard sometimes

Follow-up from squeekboard#159 (closed)

zwp_virtual_keyboard_v1@26.key(1358590892, 0, 0) doesn't always result in a keypress delivered to the application. It seems that this is happening when they keymap says that this is an ASCII letter. When the keymap specifies Uxxxx, then the event goes through.

Upstream MR: https://github.com/swaywm/wlroots/pull/1936