xwayland: assume no WM_HINTS means window wants input

Some X11 clients (e.g. Chromium, sxiv) don't set WM_HINTS. The spec
says:

> Window managers are free to assume convenient values for all fields of the
> WM_HINTS property if a window is mapped without one.

Our wlr_xwayland_icccm_input_model function assumes missing WM_HINTS
means the window doesn't want input, but this is incorrect. Assume the
window wants input unless it explicitly opts-out by setting WM_HINTS.

Closes: swaywm/sway#6107

xwayland/xwm.c

@@ -2024,7 +2024,7 @@ enum wlr_xwayland_icccm_input_model wlr_xwayland_icccm_input_model(
 		xsurface->protocols, xsurface->protocols_len,
 		WM_TAKE_FOCUS);
 
-	if (xsurface->hints && xsurface->hints->input) {
+	if (!xsurface->hints || xsurface->hints->input) {
 		if (take_focus) {
 			return WLR_ICCCM_INPUT_MODEL_LOCAL;
 		}