tree-wide: do not try to use outputs with no scene_output

- check for valid scene_output in output_is_usable()
- change many "output != NULL" checks to use output_is_usable()
- remove one now-redundant separate check for valid scene_output

Fixes a crash at startup (with autoEnableOutputs=no) due to
dereferencing null scene_output in create_output_config() since:

7d7ece21d9
("output: suppress error when output position is unavailable")

Fixes: #3357

src/menu/menu.c

@@ -731,7 +731,7 @@ menu_reposition(struct menu *menu, struct wlr_box anchor_rect)
 	/* Get output usable area to place the menu within */
 	struct output *output = output_nearest_to(menu->server,
 		anchor_rect.x, anchor_rect.y);
-	if (!output) {
+	if (!output_is_usable(output)) {
 		wlr_log(WLR_ERROR, "no output found around (%d,%d)",
 			anchor_rect.x, anchor_rect.y);
 		return;