event: handle empty subscription list in JoinSubscriptions

[forkfury]

Fix

JoinSubscriptions would block indefinitely when called with zero subscriptions. The select statement had no cases that could complete since no goroutines were spawned to send errors.

Changes

• Add early return for empty subscription list in JoinSubscriptions
• Return a subscription that waits for unsubbed channel closure, allowing proper cleanup without blocking
• Add TestMultisubEmpty to verify the fix and prevent regression

[rjl493456442]

I am not sure if it's necessary. We probably can always hold the assumption that the provided subscription list shouldn't be empty.

The change is pretty simple though.