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/xwayland.c

@@ -228,7 +228,7 @@ ensure_initial_geometry_and_output(struct view *view)
 			view->pending = view->current;
 		}
 	}
-	if (!view->output) {
+	if (!output_is_usable(view->output)) {
 		/*
 		 * Just use the cursor output since we don't know yet
 		 * whether the surface position is meaningful.