Stop sending empty subscription updates to clients

[joshua-spacetime]

It appears that eval_updates does not filter out empty subscription updates coming from eval_delta.

SpacetimeDB/crates/core/src/subscription/module_subscription_manager.rs

Line 543 in ec30c0b

let updates = eval_delta(tx, &mut metrics, plan)

We should probably update eval_delta to return None in the case of an empty update.