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/ssd/ssd-extents.c

@@ -95,7 +95,7 @@ ssd_extents_update(struct ssd *ssd)
 		wlr_scene_node_set_enabled(&ssd->extents.tree->node, true);
 	}
 
-	if (!view->output) {
+	if (!output_is_usable(view->output)) {
 		return;
 	}