[scene] Make nullentity's id nullent

While an entity with a null registry is null regardless of the id,
setting the id to nullent is useful for other purposes.

Fixes the disappearing brush models in the vulkan renderer (it uses
entity id + 1 for indexing), and prevents similar issues in the future.

include/QF/scene/entity.h

@@ -50,7 +50,7 @@ typedef struct entity_s {
 	uint32_t    id;
 } entity_t;
 
-#define nullentity ((entity_t) {})
+#define nullentity ((entity_t) { .id = nullent })
 
 typedef struct animation_s {
 	int         frame;