Fix observer callback type.

Author: OndraVoves

src/zflecs.zig

@@ -1139,7 +1139,7 @@ pub const observer_desc_t = extern struct {
     events: [FLECS_EVENT_DESC_MAX]entity_t = [_]entity_t{0} ** FLECS_EVENT_DESC_MAX,
 
     yield_existing: bool = false,
-    callback: iter_action_t,
+    callback: ?iter_action_t,
     run: ?run_action_t = null,
     ctx: ?*anyopaque = null,
     ctx_free: ?ctx_free_t = null,