fix(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